A field measuring instrument such as a differential pressure/pressure transmitter, an electromagnetic flow meter, a temperature transmitter, or the like which is used for industrial measurement includes a counter which is periodically counted up at a predetermined interval and outputs an interruption signal; a work memory which can temporarily store data; an EEPROM which can store a total operating time of the field measuring instrument and a flag indicative of an abnormality; and a processing unit for receiving the interruption signal of the counter, for incrementing the total time operating times in the work memory, for storing the incremented total operating time into the EEPROM, for self-diagnosing apparatuses in the field measuring instrument and data in accordance with self-diagnosis programs provided in the processing unit, for producing a flag indicative of a state of an abnormality when the abnormality is detected, and for storing the flag into the EEPROM together with the total operating time stored in the work memory.