A reference block 30 attached to a housing 20a is moved together with a main spindle 20 to a measurement start position A1, and the reference block 30 is so moved in a Z-axis direction as to across air injected from an air injection nozzle 25, so that a thermal displacement of the main spindle 20 is calculated based on change in pressure measured in the movement. The outer circumference of a tool mounted on the main spindle 20 is brought close to a leading end face of the air injection nozzle 25, and the air is injected to the outer circumference of the tool while rotating the tool at a predetermined position, and the air pressure is measured so that the rotational deflection of the tool is calculated based on the change in the measured pressure.