PURPOSE: To compute the size of a frame which fits the user's physical constitution quickly and accurately, by providing an operation and display device for frame sizes, which computes a seat angle, SAR, and the like, based on each detected signal that is inputted from a riding attitude detecting device, and displays the result.

CONSTITUTION: Position detectors detect a horizontal distance X1 from a hanger part 28 to a handle supporting part 23, a height Y1 from the hanger part 28 to the handle supporting part 23, a horizontal distance X2 from the hanger part 28 to a saddle supporting part 39, and a height Y2 from the hanger part 28 to the saddle supporting part 39. Then the detected signal from each position detector is sent to an A/D converter 43 through a main control box 42 and converted into the digital signal. The result is inputted and stored in an operation and display device 44. The operation and display device 44 performs the operation in accordance with programs. The operations are sequentially performed in accordance with the programs. At this stage, the kind of a bicycle, the diameter of a wheel, and the like, which are requested by the user are inputted. The size of the frame which fits the user's physical constitution is computed. The result is stored in the operation and display device 44. A frame drawing is outputted together with the computed result.