Abstract: At Step S1, the circuit forming the semiconductor integrated circuit to be designed is divided into sub circuits, and the area sizes and the positions of blocks to seat the sub circuits are determined on a chip. At Step S2, only flip-flops are counted as cells which are assumed to be the source of heat. At Step S3, a heat quantity per unit area in each block (i.e., average heat quantity) is calculated for each sub circuit. More specifically, average heat quantities are calculated from the area sizes of the blocks, the number of the flip-flops of each sub circuit, and the amount of heat generated in each flip-flop during operation of the flip-flop. Heat generated in a semiconductor integrated circuit is estimated easily by considering only a clock signal.