In a power-transmission control mechanism for controlling transmission of power from an internal combustion engine of a lawn mower to mowing blades, a controller for an electromagnetic clutch periodically repeats: reading detected engine rotational speed during a predetermined time, when an operating switch is turned on; comparing present engine rotational speed read in the present cycle with previous engine rotational speed read in the previous cycle; engaging the electromagnetic clutch when the present engine rotational speed is equal to or higher than the previous engine rotational speed; and disengaging the electromagnetic clutch when the present engine rotational speed N is lower than the previous engine rotational speed. Thus, the power of the engine is reliably transmitted to the mowing blades without occurrence of engine stall and without being influenced by the load following characteristic of the engine, the clutch, and load fluctuation.