PURPOSE: To enable automatic wiring with a wide net by allocating one of plural nets other than one net to a track to be allocated next when one net out of plural nets is allocated to a specific track.

CONSTITUTION: The number T of the track to which the net is allocated is set to '1' (SO and S3), and the function XR of the X coordinate of the track is set to '0' (S4). The net N corresponding to the masterless node positioned on the leftmost position among masterless node entered in an upper/lower restriction graph is selected (S6 and S7). When the net N is wired as a wide net (S9), a net N1 constituting the wide net N is allocated to the track T (S21) and another net N2 (net to be allocated adjacently to the net N1) constituting the wide net N is allocated to a track T+1 (S22), and the trunk of the net N1 and the trunk of the net N2, and the branch of the net N1 and the branch of the net N2 are wired respectively (S23 and S24).