To provide a power transmission device capable of precisely detecting whether a belt reaches a replacement time or not.

The power transmission device includes: a crank shaft pulley 41 disposed on a crank shaft 11; a suction cam shaft pulley 43 disposed on an intake cam shaft 22; a timing belt assembly 46 laid between the crank shaft pulley 41 and the intake cam shaft pulley 43; and a tensioner 47 which applies tension to the timing belt assembly 46 where the timing belt assembly 46 has at least two lines of a main timing belt 46a and a sub timing belt 46b in parallel, a sub-timing belt 46b is set to be easily broken as compared with the main timing belt 46a on condition that the belt is deteriorated by use, and a detection sensor 49 for detecting that the tensioner 47 is displaced by a predetermined distance or longer from an original position is disposed in the power transmission device.