To provide a power transmission device of a vehicle which transmits rotating power produced in a power source 3 to an output side through a planetary gear device 4, wherein oil can be surely supplied to a planetary gear device 4 not only when vehicle forwarding power is output but also when vehicle backing power is output.

The device includes an oil pump 7 which sucks up oil in a device case 1a, an one-way clutch 8 which does not transmit reverse rotation output of a planetary gear device 4 to the oil pump 7 while transmitting normal rotation output of the planetary gear device 4 to the oil pump 7, an oil tank 25 provided in the device case 1a and an oil path 20 which supplies oil in the oil tank 25 and oil sucked up by the oil pump 7 to the planetary gear device 4. The oil tank 25 contains a first introduction part 26 for receiving oil splashed in response to normal rotation motion of a gear 14 in the device case 1a and a second introduction part 27 for receiving oil splashed in response to the reverse rotation motion of the gear 14.

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