POWER TRANSMISSION DEVICE

[Problem] To increase the cooling efficiency of an oil.[Solution] A power transmission device (1) has a motor (2), a gear mechanism (4, 5) connected to the downstream of the motor (2), and a box (14) for housing the gear mechanism. The box (14) is formed with a first chamber (Sb) in which the gear mechanism (4, 5) and an oil (OL) are housed. The box (14) is also formed with a second chamber (CR) which is adjacent to the first chamber (Sb) and into which a cooling liquid (CL) is introduced.