A lawn mower comprising: a housing including an opened bottom; a rotation shaft extending in a vertical direction of the housing and including a lower end positioned within the housing; a cutter blade provided for the rotation shaft, and accommodated in the housing; a drive source configured to drive the cutter blade through the rotation shaft; a grass clippings container configured to store lawn grass which is cut by the cutter blade and is transported from the housing by transportation wind generated by the cutter blade; characterised in that the lawn mower comprises 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; a grass clippings container weight detection unit configured to detect weight (Wr) of the grass clippings container; and an internal pressure detection unit configured to detect internal pressure (Pr) of the housing, wherein the control unit is configured to: determine a change amount (ΔWr) per predetermined fixed time (Δt1) of the weight (Wr) detected by the grass clippings container weight detection unit; determine a change amount (ΔPr) per a predetermined fixed time (Δt2) of the internal pressure (Pr) detected by the internal pressure detection unit; and control the actuator to place the flap in a substantially horizontal state, in a case in which it is determined that two conditions are satisfied, the two conditions being a first condition where the change amount (ΔWr) of the weight (Wr) is below a predetermined weight change amount reference value (ΔWs) and a second condition where the change amount (ΔPr) of the internal pressure (Pr) has reached a predetermined internal pressure change amount reference value (ΔPs). a housing including an opened bottom; a rotation shaft extending in a vertical direction of the housing and including a lower end positioned within the housing; a cutter blade provided for the rotation shaft, and accommodated in the housing; a drive source configured to drive the cutter blade through the rotation shaft; a grass clippings container configured to store lawn grass which is cut by the cutter blade and is transported from the housing by transportation wind generated by the cutter blade; characterised in that the lawn mower comprises 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; a grass clippings container weight detection unit configured to detect weight (Wr) of the grass clippings container; and an internal pressure detection unit configured to detect internal pressure (Pr) of the housing, wherein the control unit is configured to: determine a change amount (ΔWr) per predetermined fixed time (Δt1) of the weight (Wr) detected by the grass clippings container weight detection unit; determine a change amount (ΔPr) per a predetermined fixed time (Δt2) of the internal pressure (Pr) detected by the internal pressure detection unit; and control the actuator to place the flap in a substantially horizontal state, in a case in which it is determined that two conditions are satisfied, the two conditions being a first condition where the change amount (ΔWr) of the weight (Wr) is below a predetermined weight change amount reference value (ΔWs) and a second condition where the change amount (ΔPr) of the internal pressure (Pr) has reached a predetermined internal pressure change amount reference value (ΔPs). determine a change amount (ΔWr) per predetermined fixed time (Δt1) of the weight (Wr) detected by the grass clippings container weight detection unit; determine a change amount (ΔPr) per a predetermined fixed time (Δt2) of the internal pressure (Pr) detected by the internal pressure detection unit; and control the actuator to place the flap in a substantially horizontal state, in a case in which it is determined that two conditions are satisfied, the two conditions being a first condition where the change amount (ΔWr) of the weight (Wr) is below a predetermined weight change amount reference value (ΔWs) and a second condition where the change amount (ΔPr) of the internal pressure (Pr) has reached a predetermined internal pressure change amount reference value (ΔPs).