Abstract: In an optimum placement method of circuit blocks in a semiconductor integrated circuit, from a given initial placement of circuit blocks, a new initial placement is prepared on the basis of connection information of the circuit blocks, by placing circuit blocks as many as possible, in the form of a single stream of circuit blocks in which each circuit block is connected to only an adjacent circuit block but is not connected to a not-adjacent circuit block. From the circuit blocks included in the new initial placement, there is found out a connection destination circuit block of a not-yet-placed circuit block which is not included in the new initial placement, and then, the not-yet-placed circuit block is inserted to a best position adjacent to the connection destination circuit block, thereby to temporarily determine the placement of all circuit blocks. Thereafter, only the circuit blocks in the proximity of the inserted circuit blocks is replaced to finally determine the best placement of all circuit blocks.