In a mounting angle error detection method, a relative speed to an object that has reflected continuous waves is determined. An estimated orientation of the object is determined for each frequency bin. An approximated straight line that indicates a relationship between a relative speed of a stationary object relative to the own vehicle and an orientation at which the stationary object is positioned is calculated from the relative speed and estimated orientation determined. As a mounting angle error of the onboard radar apparatus, a difference between an orientation angle identified from the approximated straight line and at which the relative speed of the stationary object relative to the own vehicle is zero, and an orientation angle at which the relative speed of the stationary object relative to the own vehicle is zero when the onboard radar apparatus is mounted in the own vehicle at a predetermined mounting angle is determined.