To measure the length of a step in walking or running.

This step length measuring instrument 3 for a treadmill 1 equipped with a belt 2 moving around in a fixed direction with its upper surface used as a tread surface is used to measure the length Ls of a step in walking or running. It comprises a rotary encoder 4 for obtaining a distance D traveled by the belt 2 and the moving speed V thereof, an infrared transmitter 6 positioned above the belt 2 to transmit infrared rays 5 in a direction normal to the moving direction of the belt 2, an infrared receiver 8 for outputting two different types of signals respectively for an infrared receiving period and for an infrared non-receiving period during which the infrared rays are interrupted by a human foot 7, and a CPU 9 for computing the length Ls of a step in walking or running from the time interval Ts of either of the two types of signals and the moving speed V of the belt 2.