A compressor that compresses a gas comprising: a cylinder that has a gas introduction compression chamber formed inside a front end thereof; a piston that is inserted into the cylinder and compresses a gas introduced into the compression chamber; a cylinder head that is attached to the front end of the cylinder and includes an internal space communicating with the compression chamber; a check valve (14, 16) that is provided in the internal space of the cylinder head; a valve pressing portion (18, 20) that presses the check valve (14, 16) so that the check valve (14, 16) does not slip off through an opening of the cylinder head connected to the internal space; and a head sealing portion that suppresses the leakage of the gas from the internal space of the cylinder head, wherein the valve pressing portion (18, 20) includes a valve pressing insertion portion (53, 70) that is inserted into the internal space of the cylinder head through the opening of the cylinder head, wherein an annular valve pressing groove portion (56, 76) is formed in an outer peripheral surface of the valve pressing insertion portion (53, 70), and wherein the head sealing portion includes a valve pressing O-ring (64, 84) that is attached into the valve pressing groove portion (56, 76) and seals a gap between the valve pressing insertion portion (53, 70) and an inner surface forming the internal space in the cylinder head, characterized by a valve pressing backup ring (66, 86) that is disposed at a position on the side of the opening with respect to the valve pressing O-ring (64, 84) inside the valve pressing groove portion (56, 76) and suppresses the movement of the valve pressing O-ring (64, 84) toward the opening wherein the valve pressing insertion portion (53, 70) includes a valve pressing insertion body (60, 80) in which an annular valve pressing groove concave portion (60a, 80a) is formed in an outer peripheral surface of a front end thereof and a valve pressing adapter (61, 81) which is separably coupled to the front end of the valve pressing insertion body (60, 80), and wherein the valve pressing adapter (61, 81) includes a valve pressing adapter outer peripheral portion that forms the valve pressing groove portion (56, 76) by covering the valve pressing groove concave portion from the front end side of the valve pressing insertion body (60, 80) while the valve pressing adapter (61, 81) is coupled to the front end of the valve pressing insertion body (60, 80). a cylinder that has a gas introduction compression chamber formed inside a front end thereof; a piston that is inserted into the cylinder and compresses a gas introduced into the compression chamber; a cylinder head that is attached to the front end of the cylinder and includes an internal space communicating with the compression chamber; a check valve (14, 16) that is provided in the internal space of the cylinder head; a valve pressing portion (18, 20) that presses the check valve (14, 16) so that the check valve (14, 16) does not slip off through an opening of the cylinder head connected to the internal space; and a head sealing portion that suppresses the leakage of the gas from the internal space of the cylinder head, wherein the valve pressing portion (18, 20) includes a valve pressing insertion portion (53, 70) that is inserted into the internal space of the cylinder head through the opening of the cylinder head, wherein an annular valve pressing groove portion (56, 76) is formed in an outer peripheral surface of the valve pressing insertion portion (53, 70), and wherein the head sealing portion includes a valve pressing O-ring (64, 84) that is attached into the valve pressing groove portion (56, 76) and seals a gap between the valve pressing insertion portion (53, 70) and an inner surface forming the internal space in the cylinder head, characterized by a valve pressing backup ring (66, 86) that is disposed at a position on the side of the opening with respect to the valve pressing O-ring (64, 84) inside the valve pressing groove portion (56, 76) and suppresses the movement of the valve pressing O-ring (64, 84) toward the opening wherein the valve pressing insertion portion (53, 70) includes a valve pressing insertion body (60, 80) in which an annular valve pressing groove concave portion (60a, 80a) is formed in an outer peripheral surface of a front end thereof and a valve pressing adapter (61, 81) which is separably coupled to the front end of the valve pressing insertion body (60, 80), and wherein the valve pressing adapter (61, 81) includes a valve pressing adapter outer peripheral portion that forms the valve pressing groove portion (56, 76) by covering the valve pressing groove concave portion from the front end side of the valve pressing insertion body (60, 80) while the valve pressing adapter (61, 81) is coupled to the front end of the valve pressing insertion body (60, 80).