A scroll compressor includes a fixed scroll member fixed to a housing and a movable scroll member engaging the fixed scroll member. Each scroll member has a base plate and a spiral wall. A compression chamber between the fixed and movable scroll members reduces in volume by orbiting the movable scroll member, thus compressing gas. A back pressure chamber is defined in the housing on a back surface side of the movable scroll base plate between the movable scroll member and the a fixed wall of the housing. An introducing passage interconnects the back pressure chamber and a relatively high pressure region. A seal member on one of the movable scroll member and the fixed wall seals the back pressure chamber as being slidable on the other. The seal member includes a portion for lowering sealing performance, which interconnects the back pressure chamber and a relatively low pressure region.