A power transmission device according to one aspect of the disclosure includes a first gear, a base member, a second gear, a support block, a position adjusting unit, and a fixing unit. The base member rotatably supports the first gear. The second gear engages with the first gear so as to be able to transmit power. The support block rotatably supports the second gear. The position adjusting unit adjusts relative positions of the support block and the base member in a direction in which an engagement depth between the second gear and the first gear varies. The fixing unit fixes the relative positions of the support block and the base member.