A work machine provided with: a work machine main body; a work implement attached to the work machine main body; a hydraulic cylinder configured to drive the work implement; a hydraulic pump configured to deliver a hydraulic fluid to drive the hydraulic cylinder; a control valve configured to control a flow of the hydraulic fluid from the hydraulic pump to the hydraulic cylinder; a delivery line configured to connect the hydraulic pump and the control valve; a pilot pump configured to output a pilot pressure to drive the control valve; a prime mover configured to drive the hydraulic pump and the pilot pump; an operation device configured to generate an operation signal to reduce the pilot pressure output from the pilot pump according to an operation to drive the control valve; and an accumulator configured to accumulate a return hydraulic fluid from the hydraulic cylinder, wherein the work machine comprising: a bypass line configured to connect the bottom fluid chamber of the hydraulic cylinder and the delivery line of the hydraulic pump by bypassing the control valve and be provided with the accumulator thereon; a pressure-accumulation control valve configured to be provided in the bypass line and between the bottom fluid chamber of the hydraulic cylinder and the accumulator; a release control valve provided in the bypass line and between the accumulator and the delivery line; characterized in that: a controller configured to control the pressure-accumulation control valve and the release control valve, wherein the controller is configured to perform control of opening the release control valve if a revolution speed of the prime mover becomes lower than a set value so that the hydraulic fluid released from the accumulator is discharged from the delivery line to the tank through the control valve, wherein the control valve is configured to switch a connection destination of the delivery line of the hydraulic pump, to connect to at least one of a bottom fluid chamber and a rod fluid chamber of the hydraulic cylinder and a tank, and if in neutral position, to connect the delivery line to the tank,. a work machine main body; a work implement attached to the work machine main body; a hydraulic cylinder configured to drive the work implement; a hydraulic pump configured to deliver a hydraulic fluid to drive the hydraulic cylinder; a control valve configured to control a flow of the hydraulic fluid from the hydraulic pump to the hydraulic cylinder; a delivery line configured to connect the hydraulic pump and the control valve; a pilot pump configured to output a pilot pressure to drive the control valve; a prime mover configured to drive the hydraulic pump and the pilot pump; an operation device configured to generate an operation signal to reduce the pilot pressure output from the pilot pump according to an operation to drive the control valve; and an accumulator configured to accumulate a return hydraulic fluid from the hydraulic cylinder, wherein the work machine comprising: a bypass line configured to connect the bottom fluid chamber of the hydraulic cylinder and the delivery line of the hydraulic pump by bypassing the control valve and be provided with the accumulator thereon; a pressure-accumulation control valve configured to be provided in the bypass line and between the bottom fluid chamber of the hydraulic cylinder and the accumulator; a release control valve provided in the bypass line and between the accumulator and the delivery line; characterized in that: a controller configured to control the pressure-accumulation control valve and the release control valve, wherein the controller is configured to perform control of opening the release control valve if a revolution speed of the prime mover becomes lower than a set value so that the hydraulic fluid released from the accumulator is discharged from the delivery line to the tank through the control valve, wherein the control valve is configured to switch a connection destination of the delivery line of the hydraulic pump, to connect to at least one of a bottom fluid chamber and a rod fluid chamber of the hydraulic cylinder and a tank, and if in neutral position, to connect the delivery line to the tank,. a bypass line configured to connect the bottom fluid chamber of the hydraulic cylinder and the delivery line of the hydraulic pump by bypassing the control valve and be provided with the accumulator thereon; a pressure-accumulation control valve configured to be provided in the bypass line and between the bottom fluid chamber of the hydraulic cylinder and the accumulator; a release control valve provided in the bypass line and between the accumulator and the delivery line; characterized in that: a controller configured to control the pressure-accumulation control valve and the release control valve, wherein the controller is configured to perform control of opening the release control valve if a revolution speed of the prime mover becomes lower than a set value so that the hydraulic fluid released from the accumulator is discharged from the delivery line to the tank through the control valve, wherein the control valve is configured to switch a connection destination of the delivery line of the hydraulic pump, to connect to at least one of a bottom fluid chamber and a rod fluid chamber of the hydraulic cylinder and a tank, and if in neutral position, to connect the delivery line to the tank,.