A method for producing a valuable substance from a raw material gas, CHARACTERIZED IN THAT the method comprises: a phase transition impurity substance removing step, wherein the raw material gas from a raw material gas generator is passed through a phase transition impurity substance remover to remove a phase transition impurity substance from the raw material gas, the phase transition impurity substance changing phases between a gas phase and a solid phase; a solid/liquid catching step, wherein the raw material gas after the phase transition impurity substance removing step is passed through a solid/liquid catcher to catch a solid or liquid impurity substance in the raw material gas; and a reacting step, wherein the raw material gas after the solid/liquid catching step is introduced to a valuable substance producing reactor, where a reaction occurs to produce the valuable substance; wherein the phase transition impurity substance removing step comprises: a first cooling step, wherein the raw material gas is passed through a first gas cooler, the raw material gas being cooled in the first gas cooler; and a phase transition impurity substance catching step, wherein the cooled raw material gas is passed through a phase transition impurity substance catcher that catches a solidified substance of the phase transition impurity substance; wherein after the phase transition impurity substance catching step and before the solid/liquid catching step, a second cooling step is performed, wherein the raw material gas is passed through a second gas cooler, where a temperature of the raw material gas is made lower than a temperature of the raw material gas in the first cooling step; and wherein a cooling target temperature of the first gas cooler is around 10 to 30 degrees C, and a cooling target temperature of the second gas cooler is around 0 to 10 degrees C. a phase transition impurity substance removing step, wherein the raw material gas from a raw material gas generator is passed through a phase transition impurity substance remover to remove a phase transition impurity substance from the raw material gas, the phase transition impurity substance changing phases between a gas phase and a solid phase; a solid/liquid catching step, wherein the raw material gas after the phase transition impurity substance removing step is passed through a solid/liquid catcher to catch a solid or liquid impurity substance in the raw material gas; and a reacting step, wherein the raw material gas after the solid/liquid catching step is introduced to a valuable substance producing reactor, where a reaction occurs to produce the valuable substance; wherein the phase transition impurity substance removing step comprises: a first cooling step, wherein the raw material gas is passed through a first gas cooler, the raw material gas being cooled in the first gas cooler; and a phase transition impurity substance catching step, wherein the cooled raw material gas is passed through a phase transition impurity substance catcher that catches a solidified substance of the phase transition impurity substance; wherein after the phase transition impurity substance catching step and before the solid/liquid catching step, a second cooling step is performed, wherein the raw material gas is passed through a second gas cooler, where a temperature of the raw material gas is made lower than a temperature of the raw material gas in the first cooling step; and wherein a cooling target temperature of the first gas cooler is around 10 to 30 degrees C, and a cooling target temperature of the second gas cooler is around 0 to 10 degrees C. a first cooling step, wherein the raw material gas is passed through a first gas cooler, the raw material gas being cooled in the first gas cooler; and a phase transition impurity substance catching step, wherein the cooled raw material gas is passed through a phase transition impurity substance catcher that catches a solidified substance of the phase transition impurity substance; wherein after the phase transition impurity substance catching step and before the solid/liquid catching step, a second cooling step is performed, wherein the raw material gas is passed through a second gas cooler, where a temperature of the raw material gas is made lower than a temperature of the raw material gas in the first cooling step; and wherein a cooling target temperature of the first gas cooler is around 10 to 30 degrees C, and a cooling target temperature of the second gas cooler is around 0 to 10 degrees C.