To provide a power transmission device that is assembled on the basis of a main pipe secured to a vehicle body and can output an input bidirectional rotation force as a unidirectional rotation force.

A driving shaft 12 is inserted into a through-hole 13 formed in an axial direction of the main pipe 11 secured to the vehicle body. A driven shaft 62 is inserted into a through-hole 63 formed in the axial direction of an auxiliary pipe 61 adjacent to the main pipe 11. Sprockets 42, 92 are provided in one ends of the driving shaft 12 and the driven shaft 62. The sprocket 42 is connected to the driving shaft 12 via a one-way clutch 4. The sprockets 42, 92 rotate in the same direction via a chain 5. A driving gear 41 is provided in the other end of the driving shaft 12. A driven gear 91 engaging with the driving gear 41 is provided in the other end of the driven shaft 62. The driving gear 41 is connected to the driving shaft 12 via a one-way clutch 3 to output a unidirectional rotation force.