A vehicle speed calculator includes a vehicle speed calculation unit for calculating a vehicle speed from a traveled distance per unit time of a feature point in a captured image shot by a camera for capturing a road surface, a reference distance mark irradiation unit and an image reference distance detection unit. The reference distance mark irradiation unit irradiates a reference distance mark to the road surface in parallel with an optical axis of the camera. The reference distance mark is formed in such a manner as to have a reference distance in a longitudinal direction of a motorcycle. The image reference distance detection unit detects an image reference distance, the longitudinal length of the reference distance mark in the shot image. The vehicle speed calculation unit calculates the vehicle speed from the traveled distance using the image reference distance and the reference distance.