An inertia measurement device, which is used in combination with a satellite positioning receiver that outputs a positioning result at every T seconds in a positioning system equipped on a vehicle, when a Z-axis angular velocity sensor, a position error P[m] based on the detection signal of the Z-axis angular velocity sensor while the vehicle moves at a moving speed V[m/sec] for T seconds satisfies Pp≥P=(V/Bz)×(1−cos(Bz×T)) (where, a bias error of the Z-axis angular velocity sensor is Bz[deg/sec] and a predetermined allowable maximum position error during movement for T seconds is Pp[m]), and a bias error Bx and By of the Y-axis angular velocity sensor satisfies Bz