An oil separator has a container main body and an flow channel. A partition wall member faces the opening of the flow channel and extends along a wall of the container main body. An upper end member seals the space between the upper end of the partition wall member and the container main body. A side end member seals a space between one side end of the partition wall member and the wall of the container main body. A gap between the partition wall member and the wall of the container main body is narrower than an inner diameter of the flow channel and is largest at an open side end. An outer circumference of the partition wall member is longer than half of the inner diameter of the flow channel and shorter than half of the circumferential length of the inner wall of the container main body.