To provide a joining method of different metals and supercharger where a compressor rotor is formed of a solid structure, the increase in the pressure ratio of the air is made possible, the output increase and the improvement of the efficiency in an internal combustion engine are attained, further, connection strength between the compressor rotor and a rotary shaft is increased, and further, even in the case the attachment/detachment between the compressor rotor and the rotary shaft is repeated, deterioration in the connected state is prevented, the reduction of the connection strength and the generation of backlash are suppressed, thus reliability is improved.

In the supercharger 1 where the tip of a rotary shaft 2 elongating from a turbine rotor 7 is provided with a compressor rotor 8, the rotary shaft is freely rotatably supported by a bearing housing 5, the turbine rotor is stored in a turbine housing 12, and the compressor rotor is stored in a compressor housing 13, the compressor rotor is composed of a disk part 8a made of an aluminum alloy and a boss part 8b made of steel or a steel alloy frictionally joined to the disk part, a screw part 22 is formed at the tip part of the rotary shaft, the screw part is screwed with the boss part, and the rotary shaft and the compressor rotor are connected.