To accurately detect a zero gravity offset in an acceleration sensor. As an embodiment of the present invention, in the case where position information can be obtained from a GPS processing section, a velocity detecting unit performs an operation according to Expression (15), using real detected acceleration .alpha.Gr that was actually obtained from the acceleration sensor, vehicle acceleration .alpha.P based on the position information, distance Dm, an amount of height change Dh based on pressure PR, and gravity acceleration g. Therefore, offset acceleration .alpha.o can be accurately calculated, and an acceleration detection signal can be converted into detected acceleration .alpha.G with high accuracy, based on a zero gravity offset value Vzgo by that the above offset acceleration .alpha.o was converted.