PURPOSE: To perform highly accurate paper thickness detection without providing any movement enlarging mechanism by converting the movement of a detection roller based on the minute displacement of the paper thickness into the displacement of frictional force to operate a switch, in a paper thickness detector in a printing machine or the like.

CONSTITUTION: At the time of such an abnormal state that the paper 21 dose not pass when the paper 21 should pass, a roller lever 24 rotates in a counterclockwise direction by a rension spring 39 when a cam follower 48 confronts a small diameter part 47b, a contact pin 40 pushes down an adjusting bolt 42, and a braking surface 31a separates from a roller 29. The roller 29 is released of its braking, and the roller arm 28 rolls the roller 29 along braking surfaces 30a and 31a by the weight 44 and a compression coil spring 46, and a switch 45 is operated to a machine base. Further, when an abnormal state that two sheets of paper pass, occurs, the detection roller 25 is pushed up to operate a switch 45 through the roller lever 24, the contact pin or the like.