To inhibit the breakage of rotary blades on involving stone etc., in mowing lawn.

The power transmission mechanism for transmitting the power from a driving source to the rotary blades is provided by having a transmission gear rotating with the rotary shaft of the rotary blades in one unit and an output gear 54 engaging with the transmission gear. The output gear 54 inserted with the output shaft 39 of the driving source can move along the axial direction of the output shaft 39, and the engaging part 56 of the output gear 54 engages with the projection part 39a of the output shaft 39. When a large burden acts on the rotary blades and driving source, the engagement between the projection part 39a of the output shaft 39 and the engaging part 56 of the output gear 54 is released and the transmission of the power from the driving source to the rotary blades is also released.