To provide a retaining mechanism capable of accurately fixing a pipe type shaft.

A rectangular hole 102 orthogonal to an axial direction is formed on the pipe type shaft 16. Meanwhile, a copper wire 114 is wound round a projection 112 by specified tension on a supporting plate 108 where the shaft 16 is inserted through a hole part 110. The wire 114 advances into the hole 102 and presses the peripheral direction end 116 of the hole 102, that is, the shaft 16 to the lower part of the hole part 110 of the plate 108. Thus, the shaft is prevented from moving up and down and its displacement in a thrust direction is restrained. Furthermore, the wire 114 abuts on the end 116 of the hole 102, thereby, the rotation of the shaft 16 is prevented. Therefore, the shaft 16 is not vibrated and is accurately fixed even when a scanning member performs scanning while it is guided by the shaft 16.