PURPOSE: To enable quick alarm upon discovery of an abnormal functioning, by detecting a faulty action by abnormality occasioned in a pulse series by installing a position detecting apparatus in which phases corresponding to specimen motions generate n pulse series of mutual difference.

CONSTITUTION: Four pulse series are output from an encoder 11 and two pulse outputs 203, 204 among them are received and firstly, a pair of rotating direction detecting means A (or B) generates regularly rotating detecting pulse outputs 235 and 237 (or 239 and 241) and, two reversely rotating detecting pulses 236 and 238 (or 240 and 242). Next, each pulse of them from the means A and B are combined in OR circuits 20, 21 and output when outputs 203W206 of the encoder 11 operate normally, an FF 26 connected to outputs 265, 266 of a U/D counter 24 through the OR circuit 25 does not generate an alarm signal 269. However, when the output 204 is abnormal and does not represent a pulse series, the FF 26 generates the signal 269. By this arrangement, an abnormal alarm is issued by stopping ignition signal or power source of a drive circuit which drives a specimen body.