To easily and efficiently carry out assembly of a scissors gear.

By an intermeshing mounting tool 60, a sub gear 20 is rotatively displaced a predetermined amount with respect to a main gear 10 against spring force of a torsion spring 30, a specific spring load in a rotating direction is applied between the main gear 10 and the sub gear 20, phases of external teeth 11 of the main gear 10 and external teeth 21 of the sub gear 20 are matched, the external teeth 11 of the main gear 10 is intermeshed with an internal teeth 62 of the intermeshing mounting tool 60 in addition to the external teeth 21 of the sub gear 20, and a matching state of the phases of the external teeth 11, 21 is maintained. In this state, external teeth 41 of an intermeshing counterpart gear 40 is intermeshed with both external teeth 11, 21 of the main gear 10 and the sub gear 20.