an inertial mouse device includes a main body ; an accelerometer unit disposed in the main body for performing a motion - sensing function of the main body with at least two degrees of freedom , and generating a calibration output value when the main body is in a still state ; and a microprocessor in communication with the accelerometer unit , receiving and processing the calibration output value with an operation so as to obtain an angle of the accelerometer unit relative to horizon . the microprocessor generates a shift signal when the main body is in a motional state , and performs calibration for the shift signal according to the angle .