PURPOSE: To enhance the sensitivity of a sense amplifier for a read only memory and simultaneously to shorten an operational cycle.

CONSTITUTION: N channel type MOS transistors 21, 22 and P channel type transistors 23, 24 are connected respectively in series between a bit line 1 and power source, the output of a NOR gate 25 receiving the potential of the bit line 1 and a grounded potential is imparted to the gates of the N channel type MOS transistors 21, 22. Then, the gates of the P channel type MOS transistors 23, 24 are connected to the N channel type MOS transistors 21, 22 side, a deciding output is generated from between the N channel type MOS transistor 22 and the P channel type MOS transistor 24.