Made up of a step of preparing a map expressing a correlation between solid phase percentage and viscosity of a slurry-form semi-solid metal (27) for a given metal composition; a step of setting a target viscosity corresponding to a target solid phase percentage using this map; a viscosity measuring step of measuring the viscosity of a semi-solid metal in a vessel (13) while cooling it; and a step of carrying out cooling until this viscosity reaches the target viscosity, by these steps being carried out in from the preparation of the map expressing the correlation between solid phase percentage and viscosity of the semi-solid metal to the end of cooling of the semi-solid metal the solid phase percentage of the semi-solid metal is made to match the target solid phase percentage. Because the viscosity is detected, the affects of cooling rate changes and time can be eliminated, and it is possible to raise management accuracy of the solid phase percentage of the semi-solid metal much further than with related art management based on time.