A lawn mower comprising: a cutter blade rotatable about a rotation shaft extending in a vertical direction; an engine configured to drive the cutter blade through the rotation shaft; a throttle valve provided for the engine; an engine speed detection unit configured to detect a rotation speed (Ner) of the engine; and a throttle opening angle detection unit configured to detect an opening angle (αr) of the throttle valve; wherein the engine has reference rotation speeds set to a predetermined first reference rotation speed and a predetermined second reference rotation speed which is higher than the first reference rotation speed by a predetermined speed; characterized by a flap provided for the cutter blade, the flap having a flap angle (θr) changeable along a horizontal line which is perpendicular to the rotation shaft; an actuator configured to control the flap angle (θr) of the flap; a control unit configured to control the actuator; wherein the control unit is configured to control the rotation speed (Ner) of the engine and the flap angle (θr) of the flap by switching operation between a first control mode and a second control mode; in the first control mode, during rotation of the engine at the first reference rotation speed, in a case in which it is determined that a change amount (ΔNer) per predetermined time (Δt1), of the rotation speed (Ner) of the engine detected by the engine speed detection unit has exceeded a predetermined reference speed change amount (ΔNs), control is implemented in a manner that the rotation speed (Ner) of the engine is maintained at the second reference rotation speed, and the actuator is controlled to increase the flap angle (θr) of the flap; and in the second control mode, during rotation of the engine at the second reference rotation speed, in a case in which it is determined that the opening angle (αr) of the throttle valve detected by the throttle opening angle detection unit has fallen below a predetermined reference opening angle (αs), control is implemented in a manner that the rotation speed (Ner) of the engine is maintained at the first reference rotation speed, and the actuator is controlled to decrease the flap angle (θr) of the flap. a cutter blade rotatable about a rotation shaft extending in a vertical direction; an engine configured to drive the cutter blade through the rotation shaft; a throttle valve provided for the engine; an engine speed detection unit configured to detect a rotation speed (Ner) of the engine; and a throttle opening angle detection unit configured to detect an opening angle (αr) of the throttle valve; wherein the engine has reference rotation speeds set to a predetermined first reference rotation speed and a predetermined second reference rotation speed which is higher than the first reference rotation speed by a predetermined speed; characterized by a flap provided for the cutter blade, the flap having a flap angle (θr) changeable along a horizontal line which is perpendicular to the rotation shaft; the control unit is configured to control the rotation speed (Ner) of the engine and the flap angle (θr) of the flap by switching operation between a first control mode and a second control mode; in the first control mode, during rotation of the engine at the first reference rotation speed, in a case in which it is determined that a change amount (ΔNer) per predetermined time (Δt1), of the rotation speed (Ner) of the engine detected by the engine speed detection unit has exceeded a predetermined reference speed change amount (ΔNs), control is implemented in a manner that the rotation speed (Ner) of the engine is maintained at the second reference rotation speed, and the actuator is controlled to increase the flap angle (θr) of the flap; and in the second control mode, during rotation of the engine at the second reference rotation speed, in a case in which it is determined that the opening angle (αr) of the throttle valve detected by the throttle opening angle detection unit has fallen below a predetermined reference opening angle (αs), control is implemented in a manner that the rotation speed (Ner) of the engine is maintained at the first reference rotation speed, and the actuator is controlled to decrease the flap angle (θr) of the flap.