A raw material gas liquefying device comprising: a feed line which feeds a raw material gas whose boiling temperature is lower than a boiling temperature of nitrogen; a refrigerant circulation line which circulates a refrigerant for cooling the raw material gas, the refrigerant circulation line including an expansion unit (37; 38) of a turbine type which expands the refrigerant to generate cryogenic energy, and an expansion unit entrance valve (21; 22) provided at an entrance side of the expansion unit (37; 38); a heat exchanger (81; 82; 83; 84; 85; 86) which exchanges heat between the raw material gas and the refrigerant; a cooler which performs initial cooling of the raw material gas and the refrigerant by heat exchange with liquid nitrogen; an expansion unit rotation speed sensor (56; 57) which detects a rotation speed of the expansion unit (37; 38); and a controller which is configured to generate an opening rate command for the expansion unit entrance valve (21; 22) so that the rotation speed of the expansion unit (37; 38) reaches a predetermined target value, and to output the opening rate command to the expansion unit entrance valve (21; 22), at start-up and stop of the expansion unit (37; 38), characterised in that the opening rate command for the expansion unit entrance valve at start-up and stop of the expansion unit is generated by performing a feedback control and that the controller is configured to generate before start-up of the expansion unit (37,38) an initial opening rate command for the expansion unit entrance valve (21; 22) so that the expansion unit entrance valve (21; 22) is opened and the refrigerant which has gone through the initial cooling and has a predetermined initial cooling flow rate which does not rotate the expansion (37; 38) unit is introduced into the expansion unit (37; 38), and to output the initial opening rate command to the expansion unit entrance valve (21; 22). a feed line which feeds a raw material gas whose boiling temperature is lower than a boiling temperature of nitrogen; a refrigerant circulation line which circulates a refrigerant for cooling the raw material gas, the refrigerant circulation line including an expansion unit (37; 38) of a turbine type which expands the refrigerant to generate cryogenic energy, and an expansion unit entrance valve (21; 22) provided at an entrance side of the expansion unit (37; 38); a heat exchanger (81; 82; 83; 84; 85; 86) which exchanges heat between the raw material gas and the refrigerant; a cooler which performs initial cooling of the raw material gas and the refrigerant by heat exchange with liquid nitrogen; an expansion unit rotation speed sensor (56; 57) which detects a rotation speed of the expansion unit (37; 38); and a controller which is configured to generate an opening rate command for the expansion unit entrance valve (21; 22) so that the rotation speed of the expansion unit (37; 38) reaches a predetermined target value, and to output the opening rate command to the expansion unit entrance valve (21; 22), at start-up and stop of the expansion unit (37; 38), characterised in that the opening rate command for the expansion unit entrance valve at start-up and stop of the expansion unit is generated by performing a feedback control and that the controller is configured to generate before start-up of the expansion unit (37,38) an initial opening rate command for the expansion unit entrance valve (21; 22) so that the expansion unit entrance valve (21; 22) is opened and the refrigerant which has gone through the initial cooling and has a predetermined initial cooling flow rate which does not rotate the expansion (37; 38) unit is introduced into the expansion unit (37; 38), and to output the initial opening rate command to the expansion unit entrance valve (21; 22).