To provide a power plant in which an internal combustion engine having a variable compression-ratio mechanism for changing the position of the rotation center-line of a crankshaft and a transmission are provided, and the center distance between a crankshaft and a main shaft of a transmission is held constant.

The power plant P comprises the internal combustion engine E having the crankshaft 6 connected with a piston 10 and a variable compression ratio mechanism R for changing the position of a rotation center line L2 of the crankshaft 6, and the transmission M including the main shaft 41 connected to the crankshaft 6 through a primary speed-reducing mechanism. The variable compression ratio mechanism R comprises a crankshaft holder 50 which rotatably supports the crankshaft 6 and is supported on the main shaft 41 so as to be rockable, and a driving mechanism 70 for rocking the crankshaft holer 50.