An oil separator, comprising: a substantially cylindrical container main body; an introduction flow channel that opens into an inner wall of said container main body, and is substantially vertically connected to said container main body; a partition wall member facing said opening of said introduction flow channel and extending along said inner wall of said container main body; an upper end member sealing a space between an upper end of said partition wall member and said inner wall of said container main body; and a side end member sealing a space between one side end of said partition wall member and said inner wall of said container main body, wherein a gap between said partition wall member and said inner wall of said container main body has a width that is not more than an inner diameter (d) of said introduction flow channel, and becomes the maximum at least at an open side end where said side end member is not provided, and wherein length of an outer circumference of said partition wall member in the horizontal direction from a position facing a center of said introduction flow channel to the open side end is longer than a half of the inner diameter (d) of said introduction flow channel and shorter than a half of circumferential length of said inner wall of said container main body, characterized in that the length of the outer circumference of said partition wall member in the horizontal direction from the position facing the center of said introduction flow channel to the open side end is longer than one sixth of the circumferential length of said inner wall of said container main body and shorter than one third of the circumferential length of said inner wall of said container main body, wherein height of said partition wall member at the open side end is longer than height thereof at the side end sealed by said side end member, and said upper end member is downwardly inclined from a part of said upper end member above said introduction flow channel toward the open side end of said partition wall member. a substantially cylindrical container main body; an introduction flow channel that opens into an inner wall of said container main body, and is substantially vertically connected to said container main body; a partition wall member facing said opening of said introduction flow channel and extending along said inner wall of said container main body; an upper end member sealing a space between an upper end of said partition wall member and said inner wall of said container main body; and a side end member sealing a space between one side end of said partition wall member and said inner wall of said container main body, wherein a gap between said partition wall member and said inner wall of said container main body has a width that is not more than an inner diameter (d) of said introduction flow channel, and becomes the maximum at least at an open side end where said side end member is not provided, and wherein length of an outer circumference of said partition wall member in the horizontal direction from a position facing a center of said introduction flow channel to the open side end is longer than a half of the inner diameter (d) of said introduction flow channel and shorter than a half of circumferential length of said inner wall of said container main body, characterized in that the length of the outer circumference of said partition wall member in the horizontal direction from the position facing the center of said introduction flow channel to the open side end is longer than one sixth of the circumferential length of said inner wall of said container main body and shorter than one third of the circumferential length of said inner wall of said container main body, wherein height of said partition wall member at the open side end is longer than height thereof at the side end sealed by said side end member, and said upper end member is downwardly inclined from a part of said upper end member above said introduction flow channel toward the open side end of said partition wall member.