u2032 that is a length of a mother board portion 17 b u2032 is set to 10 mm furthermore in the example shown in fig2 each length of the module portions 18 a u2032 and 18 b u2032 is set to 20 mm it is to be noted here that memory modules 13 a u2032 and 13 b u2032 are connected through connectors 15 u2032 to the switching circuit 20 u2032 and the mother board portions 17 u2032 b respectively the illustrated memory modules 13 a u2032 and 13 b u2032 are featured by the memory units 14 a u2032 and 14 b u2032 terminated with terminating resistors in this circuit resistance of the data bus 16 u2032 can be assumed to be essentially zero therefore the data bus 16 u2032 which serves as a distributed constant line has an impedance represented by a characteristic impedance determined by capacitance and inductance furthermore the memory units 14 a u2032 and 14 b u2032 of the respective memory modules 13 a u2032 and 13 b u2032 are terminated with impedance equal to characteristic impedance resistance of 50 u03c9 in this case moreover it is assumed that each of circuit constants is set so that impedance of the memory controller 12 u2032 seen from the switching circuit 20 u2032 may also be equal to impedance of each of the memory modules 13 a u2032 and 13 b u2032 seen from the switching circuit 20 u2032 it is also assumed that the memory controller 12 u2032 may be terminated with impedance equal to characteristic impedance when either one of the memory modules 13 a u2032 and 13 b u2032 is selected by the switching circuit 20 u2032 the above mentioned structure makes it possible to reduce to a negligible level.