PURPOSE: To provide a dimension measuring instrument which can prevent the dropping of working efficiency and, at the same time, can measure the dimension of a moving object to be measured with high accuracy by constituting the instrument so that the instrument can measure the object in a moving state.

CONSTITUTION: Of two cameras 14 and 16, the movable camera 16 can be moved in parallel with x-axis by means of a movable table 18. The interval between the cameras 14 and 16 is set at the nominal value of the dimension to be measured of an object 10 to be measured. The object 10 is carried in parallel with x-axis by means of a carrying means 12. When the object 10 enters a measuring area, a stroboscope 22 emits light. Since the light emitting period of the stroboscope 22 is as short as about 10μsec, the movement of the object 10 is negligible. Synchronously with the light emitted from the stroboscope 22, the pictures taken with the cameras 14 and 16 are frozen and fetched to a memory. After the pictures are suitably processed and the end sections of the object 10 are recognized, the dimension of the object 10 is calculated.