PURPOSE: To restrain gear noise caused by vibromotive force generated by a tooth engaging with another tooth of a gear device by actualizing a gear supporting shaft with a lessened torsional rigidity without deteriorating bending rigidity.

CONSTITUTION: A groove 2 is formed on the circumferential surface of a gear supporting shaft 1 along the circumferential direction and the part where the groove 2 of the gear supporting shaft is formed is composed so as to be supported by a bearing 3. Thus the diameter of the part where the groove 2 of the supporting shaft 1 is formed is made small and the torsional rigidity of supporting shaft 1 becomes small. The bend rigidity will not deteriorate because the supporting shaft 1 moving toward the bending direction is obstructed by the bearing 3.