The pressure leakage from the back pressure chamber installed at the back side of the movable scroll to the low pressure area can be prevented. An eccentric shaft (17) formed integrally to a drive shaft (14) is inserted into a bushing (19). A balance weight (18) is fixed to the bushing (19). A cylindrical portion (34) is provided so as to protrude at the back side of the movable scroll base (22), and the bushing (19) supports the cylindrical portion (34) via a needle bearing (21). A seal member (35) is interposed between the end surface of the cylindrical portion (34) and the balance weight (18). The inside of a cylinder of the cylindrical portion (34) is made to be a back pressure chamber (36).