Abstract: An apparatus for measuring a dimension of an object which is travelling through a measuring position comprising a conveying belt, a pair of plane mirrors each of which is arranged at each side of said conveying belt, a laser light source producing a laser light beam which is repeatedly reflected between said plane mirrors so as to form a light screen at said measuring position and a photoelectric converting device receiving the laser beam leaving said plane mirrors. The apparatus further comprises a pulse generator for producing a number of pulses a repetition rate of which is proportional to the movement of said object, a pulse counter for counting the number of said pulses for a time interval during which said object enters in said light screen and a device for calculating said dimension of the object on the basis of a count value of said pulse counter.