A liquid-proof structure of a fitting body, comprising: a first opening periphery part formed on an inside of a first fitting member; a second opening periphery part formed on an outside of the first fitting member; an opening periphery part of a second fitting member fitted to the first fitting member, the opening periphery part being fitted to a gap between the first opening periphery part and the second opening periphery part; and a buffer part defined between an inner wall surface of the second opening periphery part and an outer wall surface of the opening periphery part while the first fitting member and the second fitting member are being fitted, wherein the buffer part is a space part defined by a concave part provided on at least one of the inner wall surface of the second opening periphery part and the outer wall surface of the opening periphery part; wherein the buffer part is a space part surrounded by an outside concave part provided on the inner wall surface of the second opening periphery part and an inside concave part provided on the outer wall surface of the opening periphery part; and wherein the second opening periphery part includes: an outside base part disposed opposite to a whole or a part of an outer wall surface of the first opening periphery part in a fitting direction of the first and second fitting members (120, 130); characterized in that the second opening periphery part further includes: an outside thin wall part that forms the outside concave part with an inner wall surface (21 a1) dent more than an inner wall surface of the outside base part at a side on an opening of the second opening periphery part than the outside base part, and the opening periphery part includes an inside base part and an inside thin wall part that forms the inside concave part with an outer wall surface (31a1) dent more than an outer wall surface of the inside base part at a side on an opening of the opening periphery part than the inside base part. a first opening periphery part formed on an inside of a first fitting member; a second opening periphery part formed on an outside of the first fitting member; an opening periphery part of a second fitting member fitted to the first fitting member, the opening periphery part being fitted to a gap between the first opening periphery part and the second opening periphery part; and a buffer part defined between an inner wall surface of the second opening periphery part and an outer wall surface of the opening periphery part while the first fitting member and the second fitting member are being fitted, wherein the buffer part is a space part defined by a concave part provided on at least one of the inner wall surface of the second opening periphery part and the outer wall surface of the opening periphery part; wherein the buffer part is a space part surrounded by an outside concave part provided on the inner wall surface of the second opening periphery part and an inside concave part provided on the outer wall surface of the opening periphery part; and wherein the second opening periphery part includes: an outside base part disposed opposite to a whole or a part of an outer wall surface of the first opening periphery part in a fitting direction of the first and second fitting members (120, 130); characterized in that the second opening periphery part further includes: an outside thin wall part that forms the outside concave part with an inner wall surface (21 a1) dent more than an inner wall surface of the outside base part at a side on an opening of the second opening periphery part than the outside base part, and the opening periphery part includes an inside base part and an inside thin wall part that forms the inside concave part with an outer wall surface (31a1) dent more than an outer wall surface of the inside base part at a side on an opening of the opening periphery part than the inside base part. an outside base part disposed opposite to a whole or a part of an outer wall surface of the first opening periphery part in a fitting direction of the first and second fitting members (120, 130); characterized in that the second opening periphery part further includes: an outside thin wall part that forms the outside concave part with an inner wall surface (21 a1) dent more than an inner wall surface of the outside base part at a side on an opening of the second opening periphery part than the outside base part, and the opening periphery part includes an inside base part and an inside thin wall part that forms the inside concave part with an outer wall surface (31a1) dent more than an outer wall surface of the inside base part at a side on an opening of the opening periphery part than the inside base part.