To precisely conduct dimension measurement even when a material to be measured is moving, by a method wherein measurement signals can simultaneously be obtained from each displacement sensor while each displacement sensor is moving, and next the obtained measurement signals are performed a predetermined process at the same timing, and thereafter, desired dimension is calculated by the measurement signals which have been predeterminedly processed.

In the upper horizontal part 23a and lower horizontal part 23b of a U-shaped sensor holding member 23, a measurement part holding a pair of displacement sensors, a first pulse count type displacement sensor 21 and a second pulse count type displacement sensor 22 which are counte to each other, are provided, and a displacement amount from the displacement sensors 21, 22 is supplied as measurement signals to control means 30. The control means 30 controls input and output signals among input means 10, measurement means 20, thickness calculating means 40 and output means 50. The measurement means 20 is controlled so as to simultaneously obtain measurement signals from each of the displacement sensors 21, 22 of the measurement means 20. The signals from the measurement means 20 are detected at the same timing by a signal process part 31 to be supplied to the thickness calculating means 40.