A compression system comprising: a constant speed motor; a compressor that is configured to rotate by the constant speed motor to compress a synthesis gas including at least a first gas and a second gas to produce a compressed gas; a first line that is connected to the compressor to supply the first gas constituting a part of the synthesis gas to the compressor; a second line that is connected to the first line to supply the second gas constituting a part of the synthesis gas to the first line; a discharge line that is connected to the compressor to circulate the compressed gas discharged from the compressor; a recirculation line that connects the discharge line and the first line to recirculate a part of the compressed gas from the discharge line to the first line; a first regulating valve configured to adjust a flow rate of the first gas in the first line; a second regulating valve configured to adjust a flow rate of the second gas in the second line; a discharge valve configured to adjust a flow rate of the compressed gas in the discharge line; a recirculation valve configured to adjust a flow rate of the compressed gas in the recirculation line; a valve control device configured to perform switching between the first regulating valve, the second regulating valve, the discharge valve, and the recirculation valve based on an operation condition of the compressor, the first gas has a first molecular weight, and the second gas has a second molecular weight smaller than the first molecular weight, wherein the valve control device includes a determination unit that determines whether or not the compressor is abnormal, a motor control unit that stops an operation of the constant speed motor when the determination unit determines that the compressor is abnormal, and a switching processing unit configured to switch a fluid circulation state in the first regulating valve, the second regulating valve, the discharge valve, and the recirculation valve from a fully open state that allows circulation and that is a valve state during a rated operation of the compressor to a fully closed state that prevents circulation and that is a first valve state when the operation of the constant speed motor is stopped, and in the first valve state, the first regulating valve is switched from the fully open state to the fully closed state, the second regulating valve and the discharge valve are maintained in the fully open state, which is the valve state during the rated operation, and the recirculation valve is switched from the fully closed state, which is the valve state during the rated operation, to the fully open state. a constant speed motor; a compressor that is configured to rotate by the constant speed motor to compress a synthesis gas including at least a first gas and a second gas to produce a compressed gas; a first line that is connected to the compressor to supply the first gas constituting a part of the synthesis gas to the compressor; a second line that is connected to the first line to supply the second gas constituting a part of the synthesis gas to the first line; a discharge line that is connected to the compressor to circulate the compressed gas discharged from the compressor; a recirculation line that connects the discharge line and the first line to recirculate a part of the compressed gas from the discharge line to the first line; a first regulating valve configured to adjust a flow rate of the first gas in the first line; a second regulating valve configured to adjust a flow rate of the second gas in the second line; a discharge valve configured to adjust a flow rate of the compressed gas in the discharge line; a recirculation valve configured to adjust a flow rate of the compressed gas in the recirculation line; a valve control device configured to perform switching between the first regulating valve, the second regulating valve, the discharge valve, and the recirculation valve based on an operation condition of the compressor, the first gas has a first molecular weight, and the second gas has a second molecular weight smaller than the first molecular weight, wherein the valve control device includes a determination unit that determines whether or not the compressor is abnormal, a motor control unit that stops an operation of the constant speed motor when the determination unit determines that the compressor is abnormal, and a switching processing unit configured to switch a fluid circulation state in the first regulating valve, the second regulating valve, the discharge valve, and the recirculation valve from a fully open state that allows circulation and that is a valve state during a rated operation of the compressor to a fully closed state that prevents circulation and that is a first valve state when the operation of the constant speed motor is stopped, and in the first valve state, the first regulating valve is switched from the fully open state to the fully closed state, the second regulating valve and the discharge valve are maintained in the fully open state, which is the valve state during the rated operation, and the recirculation valve is switched from the fully closed state, which is the valve state during the rated operation, to the fully open state.