To install a hydraulic switch without installing a special oil passage, by branching an oil passage for transmission which supplies the lubricant to the transmission installed at a back of an engine, from a main oil passage, and installing the hydraulic switch on the oil the oil passage for transmission.

A power unit 40 comprising an engine 10 and a transmission, of a motor-bicycle, comprises a main gallery 43a extending along a crank shaft 57, and an oil passage for transmission 75 which is branched from the main gallery 43a and supplies the oil to the transmission, in a crank case 43, and a hydraulic switch 76 is installed on the oil passage for transmission 75 through a communication oil passage 43b. The lowering of the oil pressure of the oil passage for transmission 75 and the main gallery 43a, is detected by this hydraulic switch 76. As mentioned in the above, the projection of the hydraulic switch 76 from the engine 10 can be reduced by installing the hydraulic switch 76 inside, and a body banking angle can be increased.