A lawn mower comprising: a housing including an opened bottom; a rotation shaft extending in a vertical direction of the housing and having a lower end positioned within the housing; a cutter blade provided for the rotation shaft, and accommodated in the housing; and a belt type continuously variable transmission provided in a working power transmission system from the drive source to the rotation shaft, the belt type continuously variable transmission capable of changing a reduction ratio of a driven pulley to a drive pulley in a continuous manner, wherein the cutter blade extends along a horizontal line which is perpendicular to the rotation shaft; the driven pulley includes a fixed sheave provided for the rotation shaft, a movable sheave which is displaceable relative to the fixed sheave in an axial direction, and a torque cam mechanism configured to partially convert a rotation force applied to the movable sheave into a thrust that presses a side surface of a V-belt; and the lawn mower further comprises: a flap provided for at least part of the cutter blade, the flap having a flap angle (θr) changeable along the horizontal line; a control force converter unit configured to convert a displacement force of the movable sheave relative to the fixed sheave in the axial direction into a control force that controls the flap angle (θr) of the flap; and a transmission mechanism configured to transmit the control force of the control force converter unit to the flap, and wherein the rotation shaft comprises a hollow shaft; and the transmission mechanism is accommodated inside the hollow shaft. a housing including an opened bottom; a rotation shaft extending in a vertical direction of the housing and having a lower end positioned within the housing; a cutter blade provided for the rotation shaft, and accommodated in the housing; and a belt type continuously variable transmission provided in a working power transmission system from the drive source to the rotation shaft, the belt type continuously variable transmission capable of changing a reduction ratio of a driven pulley to a drive pulley in a continuous manner, wherein the cutter blade extends along a horizontal line which is perpendicular to the rotation shaft; the driven pulley includes a fixed sheave provided for the rotation shaft, a movable sheave which is displaceable relative to the fixed sheave in an axial direction, and a torque cam mechanism configured to partially convert a rotation force applied to the movable sheave into a thrust that presses a side surface of a V-belt; and the lawn mower further comprises: a flap provided for at least part of the cutter blade, the flap having a flap angle (θr) changeable along the horizontal line; a control force converter unit configured to convert a displacement force of the movable sheave relative to the fixed sheave in the axial direction into a control force that controls the flap angle (θr) of the flap; and a transmission mechanism configured to transmit the control force of the control force converter unit to the flap, and wherein the rotation shaft comprises a hollow shaft; and the transmission mechanism is accommodated inside the hollow shaft. a flap provided for at least part of the cutter blade, the flap having a flap angle (θr) changeable along the horizontal line; a control force converter unit configured to convert a displacement force of the movable sheave relative to the fixed sheave in the axial direction into a control force that controls the flap angle (θr) of the flap; and a transmission mechanism configured to transmit the control force of the control force converter unit to the flap, and wherein the rotation shaft comprises a hollow shaft; and the transmission mechanism is accommodated inside the hollow shaft.