While gradually increasing the widths of signal lines (104a, 104b, 105a, 105b) of first and second groups of differential signal lines (104, 105) to suppress attenuation in the lines, the opening widths of slits (104s, 105s) formed in a GND layer (102) below the differential signal lines are similarly changed. Thereby, impedance matching is realized. Further, by alternately disposing a large-width side and a small-width side of the two groups of differential signal lines (104, 105), the total wiring area widths are reduced.