A flushing circuit for a hydraulic cylinder drive circuit, comprising: a hydraulic pump connected to a tank of hydraulic oil to generate pressurized hydraulic oil; a hydraulic cylinder including a piston which slidably enters a cylinder main body and to which a piston rod is fixed and a head-side pressure chamber and a rod-side pressure chamber which are formed by the piston and the cylinder main body; a head-side in-valve supply/discharge circuit connected to the head-side pressure chamber, and having a head-side on-off valve; a rod-side in-valve supply/discharge circuit connected to the rod-side pressure chamber and having a rod-side on-off valve; a bypassing circuit having a bypass on-off valve, which connects the head-side in-valve supply/discharge circuit with the rod-side in-valve supply/discharge circuit on the upstream of the head-side on-off valve and the rod-side on-off valve; a control valve unit including: an in-valve head-side supply/discharge circuit to which a head-side supply/discharge circuit which is connected to the head-side in-valve supply/discharge circuit is connected via a head-side lowermost circuit; an in-valve rod-side supply/discharge circuit to which a rod-side supply/discharge circuit which is connected to the rod-side in-valve supply/discharge circuit is connected via a rod-side lowermost circuit; and a direction switching valve which switchably connects the in-valve head-side supply/discharge circuit and the in-valve rod-side supply/discharge circuit with the hydraulic pump and the tank; and a pump-side bypass circuit bypassing the control valve unit and including a pump-side stop valve which has an opening and closing function and connects a pump-side branch circuit branching from a pump discharge circuit of the hydraulic pump with the head-side supply/discharge circuit; and a tank-side bypass circuit bypassing the control valve unit and including a tank-side stop valve which has an opening and closing function and connects a tank-side branch circuit branching from a tank circuit connected to the tank with the rod-side supply/discharge circuit, wherein, a flushing circuit for returning discharge hydraulic oil of the hydraulic pump to the tank is constructed by the pump-side bypass circuit, the bypassing circuit, and the tank-side bypass circuit. a hydraulic pump connected to a tank of hydraulic oil to generate pressurized hydraulic oil; a hydraulic cylinder including a piston which slidably enters a cylinder main body and to which a piston rod is fixed and a head-side pressure chamber and a rod-side pressure chamber which are formed by the piston and the cylinder main body; a head-side in-valve supply/discharge circuit connected to the head-side pressure chamber, and having a head-side on-off valve; a rod-side in-valve supply/discharge circuit connected to the rod-side pressure chamber and having a rod-side on-off valve; a bypassing circuit having a bypass on-off valve, which connects the head-side in-valve supply/discharge circuit with the rod-side in-valve supply/discharge circuit on the upstream of the head-side on-off valve and the rod-side on-off valve; a control valve unit including: an in-valve head-side supply/discharge circuit to which a head-side supply/discharge circuit which is connected to the head-side in-valve supply/discharge circuit is connected via a head-side lowermost circuit; an in-valve rod-side supply/discharge circuit to which a rod-side supply/discharge circuit which is connected to the rod-side in-valve supply/discharge circuit is connected via a rod-side lowermost circuit; and a direction switching valve which switchably connects the in-valve head-side supply/discharge circuit and the in-valve rod-side supply/discharge circuit with the hydraulic pump and the tank; and a pump-side bypass circuit bypassing the control valve unit and including a pump-side stop valve which has an opening and closing function and connects a pump-side branch circuit branching from a pump discharge circuit of the hydraulic pump with the head-side supply/discharge circuit; and a tank-side bypass circuit bypassing the control valve unit and including a tank-side stop valve which has an opening and closing function and connects a tank-side branch circuit branching from a tank circuit connected to the tank with the rod-side supply/discharge circuit, wherein, a flushing circuit for returning discharge hydraulic oil of the hydraulic pump to the tank is constructed by the pump-side bypass circuit, the bypassing circuit, and the tank-side bypass circuit.