In order to reduce radiation heat from the turbine housing of a supercharger, the turbine housing is conventionally water-cooled or covered with a heat shielding material, but it is required to control heat loss due to excessive water cooling or high temperature on the outer surface of the heat shielding material. On the contrary, a solution by a cooling structure consisting of an inner thermal insulation portion of an air layer and an outer low temperature portion covering the inner thermal insulation portion has an inevitable problem of increasing number of components and upsizing. In a cooling structure of a supercharger (2) equipped with a turbine wheel (35) which rotates with exhaust gas from an engine (1) and provided, on the periphery of a turbine housing (40) for housing the turbine wheel (35), with a cooling structure (47) consisting of an inner thermal insulation portion of an air layer (45) and an outer low temperature portion covering the inner thermal insulation portion, the outer low temperature portion is constituted by integrally forming a circulation passage (46) of fresh water in a turbine cover (39) which covers and protects the turbine housing (40).