To provide a lawn mower capable of removing such an anxiety caused by conventional lawn mowers that a blade gear box may damage lawn when a mowing operation is performed on the lawn, because the blade gear box which is attached to the conventional lawn mower and used for transmitting rotating and driving force of rotary blades to swathing blades has a large thickness in the axial direction.

In this lawn mower equipped with the rotary blades 13 and a pretreatment rotor 36 located ahead of the rotary blades so that a pretreatment operation can be conducted prior to a cutting process performed by the blades, a power transmission mechanism for transmitting the rotating and driving force of the rotary blades to the pretreatment rotor is composed of a blade gear 21 which is attached to a rotary blade shaft 13a for supporting the rotary blades so as to be integrally rotated, a pretreatment rotor gear 24 which is attached to the pretreatment rotor shaft 36a for supporting the pretreatment rotor so as to be integrally rotated, an adversely rotating gear 23 always meshing with the blade gear, and a slide gear 24 always meshing with the pretreatment rotor gear and to be slidable in an axial direction, so that the pretreatment rotor can be shifted to be rotated normally to the rotary blades, to be rotated adversely or to be stopped, by performing sliding operation of the slide gear.