A power transmission device for a power-operated lawn mower of a type, in which a driving pulley and a driven pulley are respectively fitted fixedly and rotatably on a power output shaft of a prime mover such as an internal combustion engine, an electric motor, and so forth, another driven pulley is rotatably fitted on a separate shaft disposed in parallel with the output shaft, a cutting blade is fastened to the first-mentioned driven pulley rotatably fitted on the output shaft as an integral part thereof, a first drive belt is loosely extended between the driving pulley and the second-mentioned driven pulley, a second drive belt is tightly extended between the first and second-mentioned driven pulleys, a tension pulley is further provided on a swingable supporting arm to impart tension to the loosely extended belt when the cutting blade is to be rotated for lawn mowing, and a brake applying implement is provided on one part of the swingable supporting arm to apply frictional braking force to the second-mentioned driven pulley when the cutting blade is to be stopped. The tension-imparting and brake-applying operations to the loosely extended drive belt and the second driven pulley, respectively, are effected by an operator through gripping and releasing of a control lever provided at a position of a handle connected to the swingable supporting arm by a cord or wire.