PURPOSE: To improve the degree of integration by selecting one memory cell with the first and second control signals and controlling the magnetic flux quantum state of this memory cell.

CONSTITUTION: An offset current Ioff is always supplied to a memory cell 6, and the memory cell 6 is set to one of two quantum states P and Q. Negative currents -IX and -IY in opposite directions of control currents IX and IY are supplied to set the memory cell 6 to the first quantum state P in case of write of data '0' to the memory cell 6, and positive control currents +IX and +IY are supplied to the memory cell 6 to set it to the second quantum state Q in case of write of data '1'. In case of read of the memory cell 6, either polarity is selected as polarities of control currents IX and IY and they are supplied to the memory cell 6 to read it. Thus, peripheral circuits are simplified and the degree of integration is improved.