A hermetic compressor comprising: a closed vessel storing lubricating oil; an electric-driving element; and a compressing element, the compressing element including: a shaft having a main shaft rotated by the electric-driving element, and an eccentric shaft that integrally moves with the main shaft; a cylinder block having a cylindrical hole portion for forming a compression chamber, and a bearing for supporting the main shaft; a piston inserted in the cylindrical hole portion in a reciprocating manner; and a connecting device connecting the eccentric shaft and the piston; wherein the cylindrical hole portion is formed at a part corresponding to a top end of the piston to the side of the compression chamber when the piston is at the top dead point, the cylindrical hole portion having: a uniform clearance portion where an inner diameter is constant in an axial direction of the piston, wherein the cylinder block is formed such that a bottom end of the piston is exposed when the piston is at the bottom dead point, and the piston includes a concave portion recessed inward in a radial direction of the piston on its outer circumferential face, wherein at least a part of the concave portion exposes from the cylinder block when the piston is at the bottom dead point, characterised by: the cylindrical hole portion having an expanded clearance portion formed such that an inner diameter broadens from the top dead point to the bottom dead point of the piston, the expanded clearance portion being adjacent to the uniform clearance portion, and an end of the concave portion facing the uniform clearance portion and a part of the concave portion facing the expanded clearance portion when the piston is at the top dead point. a closed vessel storing lubricating oil; an electric-driving element; and a compressing element, the compressing element including: a shaft having a main shaft rotated by the electric-driving element, and an eccentric shaft that integrally moves with the main shaft; a cylinder block having a cylindrical hole portion for forming a compression chamber, and a bearing for supporting the main shaft; a piston inserted in the cylindrical hole portion in a reciprocating manner; and a connecting device connecting the eccentric shaft and the piston; a shaft having a main shaft rotated by the electric-driving element, and an eccentric shaft that integrally moves with the main shaft; a cylinder block having a cylindrical hole portion for forming a compression chamber, and a bearing for supporting the main shaft; a piston inserted in the cylindrical hole portion in a reciprocating manner; and a connecting device connecting the eccentric shaft and the piston; wherein the cylindrical hole portion is formed at a part corresponding to a top end of the piston to the side of the compression chamber when the piston is at the top dead point, the cylindrical hole portion having: a uniform clearance portion where an inner diameter is constant in an axial direction of the piston, wherein the cylinder block is formed such that a bottom end of the piston is exposed when the piston is at the bottom dead point, and the piston includes a concave portion recessed inward in a radial direction of the piston on its outer circumferential face, wherein at least a part of the concave portion exposes from the cylinder block when the piston is at the bottom dead point, characterised by: the cylindrical hole portion having an expanded clearance portion formed such that an inner diameter broadens from the top dead point to the bottom dead point of the piston, the expanded clearance portion being adjacent to the uniform clearance portion, and an end of the concave portion facing the uniform clearance portion and a part of the concave portion facing the expanded clearance portion when the piston is at the top dead point. a uniform clearance portion where an inner diameter is constant in an axial direction of the piston, wherein the cylinder block is formed such that a bottom end of the piston is exposed when the piston is at the bottom dead point, and the piston includes a concave portion recessed inward in a radial direction of the piston on its outer circumferential face, wherein at least a part of the concave portion exposes from the cylinder block when the piston is at the bottom dead point, characterised by: the cylindrical hole portion having an expanded clearance portion formed such that an inner diameter broadens from the top dead point to the bottom dead point of the piston, the expanded clearance portion being adjacent to the uniform clearance portion, and an end of the concave portion facing the uniform clearance portion and a part of the concave portion facing the expanded clearance portion when the piston is at the top dead point. the cylindrical hole portion having an expanded clearance portion formed such that an inner diameter broadens from the top dead point to the bottom dead point of the piston, the expanded clearance portion being adjacent to the uniform clearance portion, and an end of the concave portion facing the uniform clearance portion and a part of the concave portion facing the expanded clearance portion when the piston is at the top dead point.