PURPOSE: To measure the size of a gap that a moving body has during its movement on the basis of the time when a beam of light with which the moving body is irradiated passes through the gap and the moving speed, by irradiating the moving body with the beam of light.

CONSTITUTION: A front and a rear door incorporated in a car body 11 have a gap 22. The body 11 moves on a production line as shown by an arrow and its speed V is calculated on the basis of a difference in time when two beams of light emitted, for example, at certain distance pass through the car body. The car body is irradiated with spot laser emitted from a laser oscillator to generate a beam 23 of light. This beam of light is observed through an industrical television camera. The beam of light moves in the opposite direction to the array as the car body moves and when it enters the gap 22, an image of the beam of light on a screen disappears, so this time (t) is measured to calculate the size S of the gap from S=V.t.