Productivity is to be improved in assembling a semiconductor integrated circuit device. A matrix substrate is provided and semiconductor chips are disposed on a first heating stage, and then the matrix substrate is disposed above the semiconductor chips on the first heating stage. Subsequently, the semiconductor chips and the matrix substrate are bonded to each other temporarily by thermocompression bonding, while heating the chips directly by the first heating stage. Thereafter, the temporarily bonded matrix substrate is disposed on a second heating stage adjacent to the first heating stage, and then, on the second heating stage, the semiconductor chips are thermocompression-bonded to the matrix substrate, while being heated directly by the second heating stage.