POWER TRANSMISSION DEVICE

A power transmission device (1) has: a motor (2); a transmission mechanism (3) connected downstream of the motor (2); a counter gear (5) disposed downstream of the transmission mechanism (3); and a body case (9) accommodating the motor (2), the transmission mechanism (3) and the counter gear (5). The transmission mechanism (3) has a band brake (49) and an actuator (ACT) for driving the band brake (49). The body case (9) has an outer peripheral wall (147) surrounding a radial outer periphery of the transmission mechanism (3), and a side wall (143) connected to the outer peripheral wall (147) and extending radially outward from the outer peripheral wall (147). The actuator (ACT) is adjacent to the outer peripheral wall (147) and is adjacent to the side wall (143).