A scroll compressor in which a spiral body of an orbiting scroll and a spiral body of a fixed scroll are engaged with each other in a sealed case to form a compression chamber and which includes a motor for driving the orbiting scroll and a crankshaft attached as a rotary main shaft to a rotating portion of the motor, characterized in that: the crankshaft is provided with a flywheel for keeping rotation of the motor safe; the fixed scroll has a discharge port formed in its central portion, and a release valve provided on an outer circumferential side of the discharge port so as to be connected to a release port communicating with the compression chamber, and in that the scroll compresor further comprises: - a discharge head cover which is attached to a top plate of the fixed scroll to cover the discharge port and the release valve to form a discharge head space and which has a discharge valve for opening or shutting a through-hole; - a discharge guide pipe which guides a refrigerant gas within the discharge head space from the discharge head space to an outside of the sealed case; and - a solenoid valve which is connected to a suction pipe for sucking in the refrigerant gas and to the discharge guide pipe so as to communicate with the suction pipe and the discharge guide pipe and which is driven and controlled to an opened state or a shut state based on a pulse width modulation control signal; wherein the discharge head cover, the discharge guide pipe and the solenoid valve form a bypass passage for guiding the refrigerant gas within the discharge head space from the discharge guide pipe to the suction pipe when the solenoid valve is in the opened state; wherein the scroll compressor further comprises an operation instruction control means for controlling, by operation instructions, an operation of a power inverter, which supplies electric power to vary rotation speed of the motor, and operation of a solenoid drive circuit, which generates the pulse width modulation control signal to drive and control the solenoid valve; the operation instruction control means being configured to control a discharge rate by driving the motor at a predetermined set value of rotation speed not higher than a lower limit set value of rotation speed based on the motor and by driving the solenoid drive circuit. the crankshaft is provided with a flywheel for keeping rotation of the motor safe; the fixed scroll has a discharge port formed in its central portion, and a release valve provided on an outer circumferential side of the discharge port so as to be connected to a release port communicating with the compression chamber, and in that the scroll compresor further comprises: - a discharge head cover which is attached to a top plate of the fixed scroll to cover the discharge port and the release valve to form a discharge head space and which has a discharge valve for opening or shutting a through-hole; - a discharge guide pipe which guides a refrigerant gas within the discharge head space from the discharge head space to an outside of the sealed case; and - a solenoid valve which is connected to a suction pipe for sucking in the refrigerant gas and to the discharge guide pipe so as to communicate with the suction pipe and the discharge guide pipe and which is driven and controlled to an opened state or a shut state based on a pulse width modulation control signal; - a discharge head cover which is attached to a top plate of the fixed scroll to cover the discharge port and the release valve to form a discharge head space and which has a discharge valve for opening or shutting a through-hole; - a discharge guide pipe which guides a refrigerant gas within the discharge head space from the discharge head space to an outside of the sealed case; and - a solenoid valve which is connected to a suction pipe for sucking in the refrigerant gas and to the discharge guide pipe so as to communicate with the suction pipe and the discharge guide pipe and which is driven and controlled to an opened state or a shut state based on a pulse width modulation control signal; wherein the discharge head cover, the discharge guide pipe and the solenoid valve form a bypass passage for guiding the refrigerant gas within the discharge head space from the discharge guide pipe to the suction pipe when the solenoid valve is in the opened state; wherein the scroll compressor further comprises an operation instruction control means for controlling, by operation instructions, an operation of a power inverter, which supplies electric power to vary rotation speed of the motor, and operation of a solenoid drive circuit, which generates the pulse width modulation control signal to drive and control the solenoid valve; the operation instruction control means being configured to control a discharge rate by driving the motor at a predetermined set value of rotation speed not higher than a lower limit set value of rotation speed based on the motor and by driving the solenoid drive circuit.