To enhance reliability of length measurement for a moving body.

This measuring device is provided with a rear end detector 3 and front end detectors 4 for detecting a rear end 1b and a front end 1a of a moving body 1, a speed detector 5 for detecting a moving speed, a lapse time measuring instrument 10 for counting the lapse of time after the front end is detected, and a moving body-length calculating part. The device is provided with a present length calculating part 14 for calculating the length of the moving body 1 in respective times based on the number of the front end detectors 4 by which the front end is already detected at the respective times and their specified distance L, and based on a lapse time (t) after a detection time in the front end detector 4 by which the front end is detected at last at the respective times and its moving speed, a detector predicting part 17 for predicting the front end detector 4 to detect the front end next, and a detection result determining part 11 for suspending calculation operation of the moving body-length calculating part when the front end detector 4 other than the predicted front end detector 4 detects the front end.