Power transmission device

A power transmission device comprising:

• a first rotator provided with an internally toothed gear and an externally toothed gear that is further in than the internally toothed gear in a radial direction;
• a second rotator in which transmission of power from and to the first rotator is carried out;
• and
• a transmission mechanism that mediates transmission of power between the first rotator and the second rotator,
• the transmission mechanism including:
• a transmission part that sets a transmission gear ratio to a first value when the transmission mechanism engages with the internally toothed gear and sets the transmission gear ratio to a second value different from the first value when the transmission mechanism engages with the externally toothed gear, a swing part that switches an engagement object of the transmission part on a first rotator side between the internally toothed gear and the externally toothed gear, and a drive part that drives the swing part.
• wherein the transmission mechanism is a speed reduction mechanism that sets the first value to a value that makes it possible to equalize the number of revolutions of the first rotator and that of the second rotator and sets the second value to a value smaller than the first value, and
• the transmission mechanism being realized as a water pump driven by engine power.