To efficiently measure the weight of a racing horse with high accuracy.

A weight measuring instrument 1 includes a base 2, a measuring bed 5 fitted onto the base 2 so as to be vertically movable, a load sensor 10 fitted so as to touch the measuring bed 5, guide fences 15 provided on longitudinal both sides of the measuring bed 5, and a photo-microsensor 12a, 12b for detecting a racing horse H. The racing horse is guided between the guide fences 15 by pulling reins etc., and the weight of the racing horse H on the measuring bed 5 is measured with the load sensor 10, at the timing when the racing horse H passes between the photo-microsensor 12a, 12b.