In a mowing machine having an engine for driving a rotary blade via a centrifugal clutch, a shaft and a reduction gear assembly, the outer diameter of the shaft is selected such that the torsional natural vibration frequency of the rotational system extending from the engine to the rotary blade is less than one third of a rotational frequency above which the centrifugal clutch starts transmitting the engine output torque to the shaft. As a result, the torsional natural vibration frequency of the rotational system does not equal the rotational frequency or the half or one third of the rotational frequency of the engine so that undesirable resonance phenomenon will not occur.