A waterproof socket (10, 10'), comprising: a socket body (1, 51) including an insertion hole (1e, 51e) adapted to receive a straight tube lamp, the socket body (1, 51) being adapted such that a lamp pin protruding from an end cap of the straight tube lamp can be mounted to the socket body (1, 51) by rotating the straight tube lamp about a center axis thereof; a tightening cover (6, 56) adapted to partially cover the straight tube lamp when the straight tube lamp is mounted to the socket body (1, 51), wherein the tightening cover (6, 56) is tightened to the socket body (1, 51) by thread coupling about the center axis; a first internal sleeve (3, 53) making contact with the socket body (1, 51); a tubular seal (4, 54) adapted to restrain moisture from infiltrating through between the straight tube lamp and the tubular seal (4, 54), when the straight tube lamp is mounted to the socket body (1, 51); a second internal sleeve (5, 55) for accommodating the seal (4, 54) and pressing the seal (4, 54) against the first internal sleeve (3, 53) upon tightening the tightening cover (6, 56); wherein the first internal sleeve (3, 53), the seal (4, 54) and the second internal sleeve (5, 55) are arranged in the named order from the socket body (1, 51) toward the tightening cover (6, 56) and the second internal sleeve (5, 55), the seal (4, 54), and the first internal sleeve (3, 53) are adapted such that the straight tube lamp can be inserted therethrough; characterized in that the socket body (1, 51) further includes a body-side rotation restraint portion (1g, 51g) engaging with a sleeve-side rotation restraint portion (3b, 53d) formed in the first internal sleeve (3, 53) to restrain the first internal sleeve (3, 53) from rotating about the center axis; in that the first internal sleeve (3, 53) includes a first position restraint portion (3c, 53c) and the second internal sleeve (5, 55) includes a second position restraint portion (5c, 55c) engaging with the first position restraint portion (3c, 53c) of the first internal sleeve (3, 53) to restrain the second internal sleeve (5, 55) from rotating about the center axis while allowing the second internal sleeve (5, 55) to move in direction along the center axis, and in that the sleeve-side rotation restraint portion (3b, 53d) has a serrated shape, and the body-side rotation restraint portion (1g, 51g) has a serrated shape so that the body-side rotation restraint portion (1g, 51g) engages with the sleeve-side rotation restraint portion (3b, 53d) to allow the first internal sleeve (3, 53) to rotate about the center axis in one direction while restraining the first internal sleeve (3, 53) from rotating in the other direction opposite to said one direction. a socket body (1, 51) including an insertion hole (1e, 51e) adapted to receive a straight tube lamp, the socket body (1, 51) being adapted such that a lamp pin protruding from an end cap of the straight tube lamp can be mounted to the socket body (1, 51) by rotating the straight tube lamp about a center axis thereof; a tightening cover (6, 56) adapted to partially cover the straight tube lamp when the straight tube lamp is mounted to the socket body (1, 51), wherein the tightening cover (6, 56) is tightened to the socket body (1, 51) by thread coupling about the center axis; a first internal sleeve (3, 53) making contact with the socket body (1, 51); a tubular seal (4, 54) adapted to restrain moisture from infiltrating through between the straight tube lamp and the tubular seal (4, 54), when the straight tube lamp is mounted to the socket body (1, 51); a second internal sleeve (5, 55) for accommodating the seal (4, 54) and pressing the seal (4, 54) against the first internal sleeve (3, 53) upon tightening the tightening cover (6, 56); wherein the first internal sleeve (3, 53), the seal (4, 54) and the second internal sleeve (5, 55) are arranged in the named order from the socket body (1, 51) toward the tightening cover (6, 56) and the second internal sleeve (5, 55), the seal (4, 54), and the first internal sleeve (3, 53) are adapted such that the straight tube lamp can be inserted therethrough; characterized in that the socket body (1, 51) further includes a body-side rotation restraint portion (1g, 51g) engaging with a sleeve-side rotation restraint portion (3b, 53d) formed in the first internal sleeve (3, 53) to restrain the first internal sleeve (3, 53) from rotating about the center axis; in that the first internal sleeve (3, 53) includes a first position restraint portion (3c, 53c) and the second internal sleeve (5, 55) includes a second position restraint portion (5c, 55c) engaging with the first position restraint portion (3c, 53c) of the first internal sleeve (3, 53) to restrain the second internal sleeve (5, 55) from rotating about the center axis while allowing the second internal sleeve (5, 55) to move in direction along the center axis, and in that the sleeve-side rotation restraint portion (3b, 53d) has a serrated shape, and the body-side rotation restraint portion (1g, 51g) has a serrated shape so that the body-side rotation restraint portion (1g, 51g) engages with the sleeve-side rotation restraint portion (3b, 53d) to allow the first internal sleeve (3, 53) to rotate about the center axis in one direction while restraining the first internal sleeve (3, 53) from rotating in the other direction opposite to said one direction.