PURPOSE: To detect the change of an interval to be detected with high accuracy by transmitting the change to a detector composed of a diaphragm filled up with a liquid and introducing the liquid discharged from the detector to a capillary tube, and then, detecting the length of the liquid column.

CONSTITUTION: A holding body T and a transmitting element Ug are fitted to measuring points MP10 and MP20 attached to a clamping ring Kr and the holding body T has two cylinders Z which are arranged at an interval in the peripheral direction and have chambers Ka closed by a diaphragm V. When the torsional change ΔX caused by angular moment Dm generated between measuring points MP1, and MP2 is transmitted to the diaphragm V through the element Ug, a liquid column change ΔY proportional to the torsional change ΔX is generated in the correlated capillary tube K. The change ΔY is electrically detected from the short circuit of a resistor W caused by the liquid column. At the time of measuring the change ΔY, a desired magnification rate ΔY/ΔX is obtained by adjusting the ratio of the cross-sectional area of the diaphragm V to that of the capillary tube K.