In order to measure the length F of the band-shaped member 15 in all the positions in a specified width-direction region with a simple construction, a two-dimensional laser displacement sensors 18, 19 emitting laser beams 22, 23 inclined at a certain angle with respect to a longitudinal direction of the band shaped member 15 and the band-shaped member 15 move relative to each other in the longitudinal direction of the band-shaped member 15, each position of a starting end 15a and a terminating end 15b of the band-shaped member 15 crossing the laser beam 22, 23 is detected repeatedly by the two-dimensional laser displacement sensors 18, 19 each time a relative moving distance between the two-dimensional laser displacement sensors 18, 19 and the band-shaped member 15 reaches a certain value, a position information of the starting end 15a and the terminating end 15b of the band-shaped member 15 at each width-direction position is obtained from a detection result thereof, and the length of the band-shaped member 15 at each width-direction position is obtained based on the position information of the starting end 15a and the terminating end 15b at each width-direction position. Thereby, it is possible to obtain the length of the band-shaped member 15 at any width-direction position in the width-direction region crossing the laser beam 22, 23.