A correction device determines, by calculation, a real acceleration based on output from a velocity sensor 4. GPS mediated signal receiving portion 2 receives radio navigation waves from plural GPS satellites. The device detects a frequency change of the radio waves evoked as a result of Doppler effect, acquires horizontal and vertical velocity components based on the frequency change, and determines, by calculation, a gradient angle. Then, the device determines, by calculation, the gravitational acceleration sine of the gradient angle (G. sin ). The device adds the real acceleration and the gravitational acceleration sine of the gradient angle (G. sin ) to provide a theoretical acceleration of the acceleration sensor. The device divides the theoretical acceleration of the acceleration sensor with the sensitivity of the acceleration sensor 6, and subtracts the result from output from the acceleration sensor 6, to provide a right offset value. Then, the sequentially obtained offset values are averaged to give an updated offset value. This process makes it possible to readily obtain a right offset value even during the driving of the mobile body.