To ensure the linearity of a measurement signal and to reduce the chance for changing a device when exchanging a lot by virtually constantly retaining the contact force of a movable detection mechanism over the total measurement range of the detection mechanism when pressing the detection mechanism to a sliver.

An adjustment mechanism 30 operates as a capacity accumulator which is activated by liquid pressure or air pressure and consists of cylinder spaces 32 and 34 provided with a piston 33. A compression spring 35 is arranged in the cylinder space 34. The operation of the force of the spring 35 is put in order so that the piston 33 is moved in the direction of the cylinder space 32. Also, the capacity accumulator is connected to a pressure generation device 38 which can be braked via a connection part 37. A dimension is determined so that pressure fluctuation generated over the entire surface of the connection part from a detection roll 40' to the piston 33 can be relaxed to enable the pressure within the cylinder space 32 to be retained at a virtually constant numeric value.