PURPOSE: To make self-diagnosis with a simple device by providing a discriminator which selects the value having the smaller difference with the memorized previous measuring values when the difference between the measuring values determined by a pair of elements from a part to be measured exceeds a prescribed value.

CONSTITUTION: A pair of data are determined by a pair of the measuring elements 31, 32 from the part to be measured. A difference between a pair of the data is determined by a comparator 33 and a signal indicating that either is defective is fed to the defect discriminator 34 when said difference is larger than the preliminarily set certain value. The discriminator 34 compares the previous data stored in a memory 35 and the data of this time with respect to the same measuring element, discriminates the system of the element having the larger difference as a defect and adopts the data having the smaller difference as normal data. The normal data is fed through a line L to an output part 41. The self- diagnosis is thus made with the simple device and the defective data are eliminated.