Headrest support structure of conveyance seat

1. A headrest support structure of a conveyance seat, comprising:

an upper frame that is provided with a pair of fitting holes and that constitutes a back frame; and

a pair of cylindrical headrest holders, each of which is configured to fit in each of the pair of fitting holes to be fixed to the upper frame, wherein

a transverse cross-section of a fitting portion of each headrest holder configured to fit in each of the corresponding fitting holes has an asymmetric shape about a fitting axis parallel to a fitting direction in which the headrest holder fits in the fitting hole, the transverse cross-section being a cross-section in a direction orthogonal to the fitting axis,

the pair of fitting holes are oriented differently relative to each other and when the asymmetric shapes of each headrest holder are oriented in a same initial position about the respective fitting axis of each headrest holder, the asymmetric shapes of each of the pair of headrest holders is configured to fit in one of the pair of fitting holes, and the asymmetric shapes of each of the pair of headrest holders is configured to fit in the other of the pair of fitting holes only when the pair of the headrest holders are at least one of:

inverted in a top-bottom direction of the conveyance seat with respect to the initial position; and

rotated about the fitting axis of the respective headrest holder with respect to the initial position.

2. The headrest support structure of the conveyance seat according to claim 1 , wherein the transverse cross-section of the fitting portion has a shape in which one corner of a rectangle is changed such that two sides connected to the one corner are joined with a bridging portion.

3. The headrest support structure of the conveyance seat according to claim 1 , wherein the transverse cross-section of the fitting portion has a trapezoidal shape having two adjacent right angles, an obtuse angle, and an acute angle.

4. The headrest support structure of the conveyance seat according to claim 1 , wherein the pair of fitting holes is provided such that front and rear sides of each headrest holder when the headrest holder fits in one fitting hole are the front and rear sides of the headrest holder when the headrest holder fits in the other fitting hole.

5. The headrest support structure of the conveyance seat according to claim 1 , further comprising a pair of headrest supports, each of which fits in one of the pair of headrest holders, wherein

the pair of fitting holes is provided such that each headrest support fits in the corresponding headrest holder.

6. The headrest support structure of the conveyance seat according to claim 1 , wherein the pair of fitting holes is provided to be line symmetrical to each other.

7. The headrest support structure of the conveyance seat according to claim 1 , wherein the fitting portion has a plurality of corners, and one of the plurality of corners is a different shape from the other corners.

8. The headrest support structure of the conveyance seat according to claim 1 , wherein each headrest holder is provided such that an orientation of the transverse cross-section of the fitting portion is changed only when the headrest holder is one of inverted in a top-bottom direction of the conveyance seat and rotated about the fitting axis.