A check valve (8, 9) being inserted into a through-hole of a container main body of a substrate container containing a substrate, and controlling gas flowing from and to the interior and the exterior of the substrate container, wherein, when an end of the check valve (8, 9) located inside the container main body is defined as a front end: the check valve (8, 9) has a cylindrical first housing and a cylindrical second housing mounted on the through-hole, a valve disc (13, 23) contained in the first housing and the second housing, and a fixing ring that is mounted on the rear end side of the first housing in the manner of being rotatable relative to the first housing and fixes the first housing to the container main body from rearward; the first housing is installed on the front end side of the second housing and the second housing is inserted into the first housing at the front end and attached to the first housing; the first housing has a fixing piece protruding rearward from the rear end and a fixing claw being formed at the rear end of the fixing piece and fixing the fixing ring; the fixing ring has a cylindrical sidewall mounted outside the first housing and a flange protruding outward from the sidewall; on the inner circumferential face of the sidewall, a guide recess extending in the axial direction and guiding the fixing claw, a detent protruding inward and abutting on the fixing piece, and a hook bent from the detent in the circumferential direction and locked to the fixing piece are installed; the second housing has a folded flange being folded back from the front end side and abutting on the inner circumferential face of the first housing all around the circumference; and between the inner circumferential face of the first housing and the folded flange, a locking mechanism (12d, 14b) to lock the second housing to the first housing is installed. the check valve (8, 9) has a cylindrical first housing and a cylindrical second housing mounted on the through-hole, a valve disc (13, 23) contained in the first housing and the second housing, and a fixing ring that is mounted on the rear end side of the first housing in the manner of being rotatable relative to the first housing and fixes the first housing to the container main body from rearward; the first housing is installed on the front end side of the second housing and the second housing is inserted into the first housing at the front end and attached to the first housing; the first housing has a fixing piece protruding rearward from the rear end and a fixing claw being formed at the rear end of the fixing piece and fixing the fixing ring; the fixing ring has a cylindrical sidewall mounted outside the first housing and a flange protruding outward from the sidewall; on the inner circumferential face of the sidewall, a guide recess extending in the axial direction and guiding the fixing claw, a detent protruding inward and abutting on the fixing piece, and a hook bent from the detent in the circumferential direction and locked to the fixing piece are installed; the second housing has a folded flange being folded back from the front end side and abutting on the inner circumferential face of the first housing all around the circumference; and between the inner circumferential face of the first housing and the folded flange, a locking mechanism (12d, 14b) to lock the second housing to the first housing is installed.