Disclosed is a mechanism of a four-wheel drive vehicle, for enabling an engine and a transmission to be arranged compact by using an empty space within a body frame of the vehicle. The transmission includes a power take-off part for driving front wheels of the vehicle, and a rear reduction gear mechanism for driving rear wheels of the vehicle. The transmission and the engine are mounted on a swing arm which is supported on the body frame of the vehicle so as to be able to swing in an up and down direction.