An on-vehicle radar apparatus includes a radar sensor and a mounting angle calculation section that calculates a mounting angle of the radar sensor, and the radar sensor is mounted on a vehicle so that a sensing area includes a direction of 90 degrees relative to a front-back direction of the vehicle and detects a relative speed to an observation point at which the radar wave is reflected in the sensing area and an azimuth at which the observation point is located. The mounting angle calculation section calculates a mounting angle of the radar sensor from an azimuth of a speed zero observation point, the speed zero observation point being the observation point with a relative speed of zero.