To provide a thickness measuring device capable of improving the resolution in the width direction of a measured object by finely squeezing the radiated beam from a head in the width direction of the measured object.

The electromagnetic wave beam from the radiation source side head fitted to a scanning mechanism is radiated to the measured object, and the transmitted beam transmitted and attenuated according to the thickness or basic weight of the measured object is detected by a detector side head to measure the thickness of the measured object in this thickness measuring device. The detector side head is provided with a means arranged with a plurality of detecting elements in the width direction of the measured object to finely squeeze the radiated beam to the measured object in the width direction of the measured object and a processing device extracting only the signal meeting the prescribed reference within the output of the detector side head to obtain the thickness of the measured object based on it.