In an apparatus for calibrating an inertial sensor configured to detect an angular velocity of a vehicle, an angular velocity estimator is configured to estimate a value for an angular velocity of the vehicle based on a value for an attitude of the vehicle and a value for a change in attitude of the vehicle estimated by the attitude estimator, and an error estimator is configured to estimate a value for an angular velocity error that is an error between the value for the angular velocity of the vehicle estimated by the angular velocity estimator and the value for the angular velocity of the vehicle detected by the inertial sensor. A calibrator is configured to calibrate the value for the angular velocity of the vehicle detected by the inertial sensor based on the value for the angular velocity error estimated by the error estimator.