This power transmission device (10) is provided with a motor (12), a worm (22) that is connected to a rotating shaft (21) of the motor (12), and a worm wheel (23) that engages with the worm (22) and is disposed on an operation pedal side relative to the worm (22), wherein a biasing member (51), which biases the motor (12) toward a direction in which the worm (22) and the worm wheel (23) make contact during driving of the motor (12), is disposed between the motor (12) and a housing (11).