To attain quickly and highly-accurate caster angle measurement and wheel alignment measurement by quantizing an angle of a line to be measured to a reference line, detecting them, and outputting a value at the time of changing as an effective caster angle.

This wheel alignment measuring device consists of a measuring plate 4 and an image unit 5 involving a color CCD camera. The plate 4 is provided with the first circle mark MC1 whose center is an origin, the second circle marks MC2 which are disposed in a rectangular grid form, and caster angle detecting lines CL disposed at equal intervals in parallel to one direction of the grid. The detecting line CL is extracted by image-processing data of the camera, and caster angle data is obtained as a step value by using virtual reference lines which are preset. By means of data processing with wheel alignment measurement using the caster angle data at the time of changing as an effective caster data, it is thus possible to conduct prompt measurement with prescribed measurement precision.