By first readout, the current input from a selected cell is converted by a preamplifier and a VCO into pulses of a frequency inversely proportionate to the current value, and the number of the pulses within a preset time interval is counted by a counter 5 so as to be stored in a readout value register. A selected cell is then written to one of two storage states, and second readout is then carried out. The storage state of the selected cell is verified by comparing a count value of the counter for the second readout, a count value for the first readout as stored in a readout value register and a reference value stored in a reference value register to one another. By the use of the VCO, the integrating capacitor for the current or the generation of a reference pulse may be eliminated.