PURPOSE: To obtain a lightweight crank shaft at low cost by forming a main shaft member, a eccentric shaft member and a balance weight portion integrally from synthetic resin, and embedding a metal material in the balance weight portion.

CONSTITUTION: A cranck shaft 5 for a compressor consists of a main shaft member 8 connected to the rotor of an electric motor, a crank pin portion 9 connected to a compressor portion, and a balance weight portion 11 having a thrust face 10 on its lower face, and an oil hole 12 and an oil feed pump portion 13 are formed and all of them are integrally molded from synthetic resin. During injection molding, a metal material 14 is inserted into the balance weight portion 11. Thereby the cranck shaft does not require processes such as finish grinding and drilling, etc., and is lightweight and low-costed.