An interconnection network is provided with switch devices for computers for a plurality of computers, respectively, a predetermined number of physical lines for switch devices for computers are bound to form each of logical lines, the physical lines forming the logical lines interconnect the other switch devices for computer via a plurality of switch devices for relay to form a parallel computer system. When a data transmission speed required between the computers is judged to be lower than a line transmission speed determined by the number of physical lines of any switch device for relay, switch setting-off unit turns off power of the whole switch device for relay that has been judged to shut off the switch device for relay from the interconnection network.