A motor-driven compressor characterized by comprising: a compression mechanism unit including a movable scroll and a fixed scroll operative to compress a refrigerant discharged from a suction pressure region (121, 211, 24h, 32, 221h, 31), wherein the movable scroll and the fixed scroll defines a compression chamber having a volume that is decreased by an orbiting motion of the movable scroll; a rotation shaft; an electric motor that drives the movable scroll with the rotation shaft; a housing (11 to 13) that accommodates the compression mechanism unit and the electric motor; an opposing member (21, 24) arranged in the housing (11 to 13) and opposed to the movable scroll, wherein the opposing member (21, 24) is located at a side of the movable scroll opposite to the fixed scroll, the opposing member (21, 24) includes an opposing end face (21b, 24b), which is opposed to the movable scroll, and the movable scroll includes a movable end face, which is opposed to the opposing member (21, 24); a back pressure region (62, 27a) located at a side of the movable scroll proximate to the opposing member (21, 24), wherein the back pressure region (62, 27a) is configured so that a pressure of the refrigerant in the back pressure region (62, 27a) is operative to apply a force to the movable scroll, and the force is operative to push the movable scroll against the fixed scroll; characterized by a defining portion arranged in the movable end face, wherein the defining portion contacts the opposing end face (21b, 24b) and defines the back pressure region (62, 27a) and the suction pressure region (121, 211, 24h, 32, 221h, 31); wherein the orbiting motion of the movable scroll moves the defining portion, the opposing member (21, 24) includes a communicating portion (21e, 24a), and when the orbiting motion of the movable scroll moves the defining portion, the communicating portion (21e, 24a) intermittently communicates the back pressure region (62, 27a) and the suction pressure region (121, 211, 24h, 32, 221h, 31). a compression mechanism unit including a movable scroll and a fixed scroll operative to compress a refrigerant discharged from a suction pressure region (121, 211, 24h, 32, 221h, 31), wherein the movable scroll and the fixed scroll defines a compression chamber having a volume that is decreased by an orbiting motion of the movable scroll; a rotation shaft; an electric motor that drives the movable scroll with the rotation shaft; a housing (11 to 13) that accommodates the compression mechanism unit and the electric motor; an opposing member (21, 24) arranged in the housing (11 to 13) and opposed to the movable scroll, wherein the opposing member (21, 24) is located at a side of the movable scroll opposite to the fixed scroll, the opposing member (21, 24) includes an opposing end face (21b, 24b), which is opposed to the movable scroll, and the movable scroll includes a movable end face, which is opposed to the opposing member (21, 24); a back pressure region (62, 27a) located at a side of the movable scroll proximate to the opposing member (21, 24), wherein the back pressure region (62, 27a) is configured so that a pressure of the refrigerant in the back pressure region (62, 27a) is operative to apply a force to the movable scroll, and the force is operative to push the movable scroll against the fixed scroll; characterized by a defining portion arranged in the movable end face, wherein the defining portion contacts the opposing end face (21b, 24b) and defines the back pressure region (62, 27a) and the suction pressure region (121, 211, 24h, 32, 221h, 31); wherein the orbiting motion of the movable scroll moves the defining portion, the opposing member (21, 24) includes a communicating portion (21e, 24a), and when the orbiting motion of the movable scroll moves the defining portion, the communicating portion (21e, 24a) intermittently communicates the back pressure region (62, 27a) and the suction pressure region (121, 211, 24h, 32, 221h, 31).