PURPOSE: To enable measurement of a resistance with the accuracy close to a reference resistance, by measuring time taken to accumulate electric charge in a charge storage element up to a certain voltage with the reference resistance only on one hand and with the reference resistance set in parallel with an element to be measured to determine the ratio of the separate results.

CONSTITUTION: A Tr1 is turned OFF to enter a reference resistance measuring mode and a capacitor C is discharged completely. A Tr2 is turned OFF by setting an output port 2 to 0 to start the charging of the capacitor with a reference resistance 12 only. A CPU 1 reads from an input port 5 at each fixed time and when a value read is 0, the increment of a TIMCNT is expected. When a terminal voltage vc of the capacitor C exceeds a reference voltage Vth, with the inversion of a voltage comparator 7, the input port 5 is turned to 1 stop the increment of the TIMCNT. Then, the Tr1 is turned ON to enter a temperature measuring mode and a thermistor 11 is connected in parallel to the reference resistance 12 to perform a measurement as mentioned above. The ratio of separate counts is determined and multipled by RS to obtain a resistance value RT to be measured.