A construction machine comprising: a pilot pump constituting a pilot hydraulic source together with a tank ; a pressure reducing valve type pilot valve connected to a pilot delivery line of the pilot pump and reducing a pressure of a pilot pressure oil supplied from the pilot delivery line and outputting a pilot pressure to a directional control valve on a main line side ; and a gate lock valve provided between the pilot pump and the pressure reducing valve type pilot valve and switching the pressure in the pilot delivery line to either one of a high pressure state by a delivery pressure of the pilot pump or a low pressure state connected to the tank in accordance with an operation of a gate lock lever , the pilot delivery line including a throttle disposed between the pilot pump and the gate lock valve and limiting a flowrate of the pilot pressure oil delivered from the pilot pump ; characterised by a check valve disposed between the gate lock valve and the pressure reducing valve type pilot valve and allowing a flow of the pilot pressure oil from the pilot pump toward the pressure reducing valve type pilot valve and shutting down the flow in an opposite direction; a bypass line having one end side connected to the pilot delivery line between the pilot pump and the throttle and the other end side connected to the pilot delivery line between the check valve and the pressure reducing valve type pilot valve so as to bypass the throttle , the gate lock valve , and the check valve ; a lock switching valve provided in the bypass line and normally shutting down a flow of the pilot pressure oil from the pilot pump into the bypass line and allowing the flow of the pilot pressure oil through the bypass line when a pressure generated in the pilot delivery line exceeds a predetermined pressure between the gate lock valve and the check valve . a pilot pump constituting a pilot hydraulic source together with a tank ; a pressure reducing valve type pilot valve connected to a pilot delivery line of the pilot pump and reducing a pressure of a pilot pressure oil supplied from the pilot delivery line and outputting a pilot pressure to a directional control valve on a main line side ; and a gate lock valve provided between the pilot pump and the pressure reducing valve type pilot valve and switching the pressure in the pilot delivery line to either one of a high pressure state by a delivery pressure of the pilot pump or a low pressure state connected to the tank in accordance with an operation of a gate lock lever , the pilot delivery line including a throttle disposed between the pilot pump and the gate lock valve and limiting a flowrate of the pilot pressure oil delivered from the pilot pump ; characterised by a check valve disposed between the gate lock valve and the pressure reducing valve type pilot valve and allowing a flow of the pilot pressure oil from the pilot pump toward the pressure reducing valve type pilot valve and shutting down the flow in an opposite direction; a bypass line having one end side connected to the pilot delivery line between the pilot pump and the throttle and the other end side connected to the pilot delivery line between the check valve and the pressure reducing valve type pilot valve so as to bypass the throttle , the gate lock valve , and the check valve ; a lock switching valve provided in the bypass line and normally shutting down a flow of the pilot pressure oil from the pilot pump into the bypass line and allowing the flow of the pilot pressure oil through the bypass line when a pressure generated in the pilot delivery line exceeds a predetermined pressure between the gate lock valve and the check valve . <EOS>