Image forming apparatus including shifted roller

1. An image forming apparatus comprising:

a belt member stretched across at least two rollers, each of the at least two rollers including a first end portion and a second end portion;

a protrusion disposed, such that it protrudes along the traveling direction of the belt member, on an inner surface of one edge side of the belt member;

an engagement portion that is formed in the first end portion of the rollers and with which the protrusion is engageable; and

a plurality of image carriers arranged in a line, wherein each image carrier carries a toner image transferred to a recording medium conveyed by the belt member or the surface of the belt member,

wherein the second end portion of one roller of the at least two rollers is shifted with respect to the first end portion of the one roller in a direction away from and orthogonal to the line of the image carriers, and the second end portion of an other roller of the at least two rollers is not shifted with respect to the first end portion of the other roller in a direction away from the image carrier; and

wherein the second end portion of one roller is drawn away from the line of the plurality of image carriers and the first end portion of the one roller is not drawn away from the line of the plurality of rollers.

2. The image forming apparatus of claim 1 , wherein the one roller is a roller configuring a contact surface of the belt member that contacts the image carriers.

3. The image forming apparatus of claim 1 , wherein the one roller is a non-driven roller.

4. The image forming apparatus of claim 3 , wherein a minimum distance between a center position serving as a rotational reference position when the non-driven roller shifts and an axial center line of a drive roller is constant.

5. The image forming apparatus of claim 1 , wherein the one roller is axially supported on a casing.

6. The image forming apparatus of claim 1 , wherein the first end portion of the one roller is adjacent to the line of the image carriers and the second end portion of the one roller is separated from the line of the image carriers.

7. An image forming apparatus comprising:

a belt conveyance unit including

a belt member that is stretched across at least two rollers and conveys a recording medium, each of the at least two rollers including a first end portion and a second end portion,

a protrusion disposed, such that it protrudes along the traveling direction of the belt member, on an inner surface of one edge side of the belt member,

an engagement portion that is formed in the first end portion of the rollers and with which the protrusion is engageable, and

a casing that axially supports the rollers;

a body frame including an image carrier for transferring an image to the recording medium; and

a cover to which the belt conveyance unit is attached, is pivotably attached to the body frame, and opens and closes off the image carrier,

wherein when the cover pivots toward the body frame and closes off the image carrier, the casing contacts the body frame, whereby the second end portion of one roller of the at least two rollers is shifted with respect to the first end portion of the one roller in a direction away from the image carrier.

8. The image forming apparatus of claim 7 , wherein a convex portion is formed on the casing or the body frame and causes the casing to bend, whereby the second end portion of the one roller is shifted in the direction where it separates from the image carrier.

9. The image forming apparatus of claim 7 , wherein the one roller is a roller configuring a contact surface of the belt member that contacts the image carrier.

10. The image forming apparatus of claim 7 , wherein the one roller is a non-driven roller.

11. The image forming apparatus of claim 10 , wherein a minimum distance between a center position serving as a rotational reference position when the non-driven roller shifts and an axial center line of a drive roller remains constant.