Image forming apparatus

1. An image forming apparatus performing an image formation by transferring a toner image on a sheet, the image forming apparatus comprising:

a cassette attachment part to which a sheet feeding cassette storing the sheet is detachably attached; and

a collection side attachment part disposed adjacent to the cassette attachment part and to which a collection container storing a waste toner discharged without transferred on the sheet is detachably attached, wherein

the cassette attachment part includes:

a cassette rail which supports the sheet feeding cassette slidably; and

a rail holding member which holds the cassette rail,

the collection side attachment part includes a displacement sensor which is disposed so as to face the collection container and detects an electrostatic capacity varying in response to an accumulation amount of the waste toner stored in the collection container, and

the rail holding member is disposed so as to face at least a part of the displacement sensor.

2. The image forming apparatus according to claim 1 , wherein

the displacement sensor detects a variation in electric field between a transmission electrode and a reception electrode which are disposed so as to face each other across the collection container, and

one of the transmission electrode and the reception electrode is disposed so as to face the rail holding member.

3. The image forming apparatus according to claim 2 , wherein

the transmission electrode is disposed on an opposite side to the cassette attachment part with respect to the collection container.

4. The image forming apparatus according to claim 1 , wherein

the displacement sensor detects a variation in electric field between a transmission electrode and a reception electrode which are disposed between the collection container and the rail holding member, and

at least one of the transmission electrode and the reception electrode is disposed so as to face the rail holding member.

5. The image forming apparatus according to claim 4 , wherein

the transmission electrode and the reception electrode are mounted on a single board side by side in an upper-and-lower direction.

6. The image forming apparatus according to claim 1 , wherein

the collection container includes:

a cylindrical container main body having a spiral conveyance rib protruding inward in a radial direction from an outer circumferential face, and

a cap which supports an end portion of the container main body on one side in an axial direction so as to be rotatable around the axis and has a communication port communicating with an inside of the container main body,

the collection side attachment part supports the cap not rotatably and the container main body rotatably,

when the container main body attached to the collection side attachment part is rotated around the axis, the waste toner introduced into the inside of the container main body through the communication port is conveyed toward the other side in the axial direction, and

at least a part of the displacement sensor is disposed between the container main body and the rail holding member on the other side from an axial center of the container main body.

7. The image forming apparatus according to claim 6 , wherein

the reception electrode is disposed between the container main body and the rail holding member.