A main shaft device (10) is attached to a work machine (80). The main shaft device (10) is provided with a main shaft (14) that supports a tool (18) or a workpiece, and a main shaft motor (35) that causes the main shaft (14) to rotate. The main shaft device (10) is provided with bearings (16a, 16b, 17) that are rolling bearings which support the main shaft in an inner race, and a housing (12) anda rear housing (13) that secure an outer race of the bearings (16a, 16b, 17). The housing (12) and the rear housing (13) are formed of a material having a coefficient of thermal expansion greater thanthat of the main shaft (14).