A rotary compressor comprising: a cylinder; an eccentric portion of a shaft disposed in the cylinder; a piston fitted to the eccentric portion; a slot provided in the cylinder; a groove formed in the piston; and a vane having a columnar portion on its one of ends; in which a compression chamber is formed between the cylinder and the piston the columnar portion is swingably fitted into the groove, the vane reciprocates in the slot as the shaft rotates, and the compression chamber is divided into a high pressure-side space and a low pressure-side space by the vane, wherein the groove is formed into a columnar shape having an arc whose angle is greater than 180°, and a fictitious extension (La) of the arc is located inside of an outer periphery fictitious line (Lb) of the piston; characterised in that a body of the vane includes one of side surfaces facing the low pressure-side space, the other side surface facing the high pressure-side space, an upper end surface which is connected to an upper end of the one side surface and an upper end of the other side surface, and a lower end surface which is connected to a lower end of the one side surface and a lower end of the other side surface, a low pressure-side constriction is provided between the one side surface and the columnar portion, a high pressure-side constriction is provided between the other side surface and the columnar portion, in a state where the piston is in contact with the slot, a top clearance volume (Vg) is formed by the high pressure-side constriction, the piston and the cylinder, a clearance volume (Vf) is formed by the low pressure-side constriction, the piston and the cylinder, and the top clearance volume (Vg) is smaller than the clearance volume (Vf). a cylinder; an eccentric portion of a shaft disposed in the cylinder; a piston fitted to the eccentric portion; a slot provided in the cylinder; a groove formed in the piston; and a vane having a columnar portion on its one of ends; in which a compression chamber is formed between the cylinder and the piston the columnar portion is swingably fitted into the groove, the vane reciprocates in the slot as the shaft rotates, and the compression chamber is divided into a high pressure-side space and a low pressure-side space by the vane, wherein the groove is formed into a columnar shape having an arc whose angle is greater than 180°, and a fictitious extension (La) of the arc is located inside of an outer periphery fictitious line (Lb) of the piston; characterised in that a body of the vane includes one of side surfaces facing the low pressure-side space, the other side surface facing the high pressure-side space, an upper end surface which is connected to an upper end of the one side surface and an upper end of the other side surface, and a lower end surface which is connected to a lower end of the one side surface and a lower end of the other side surface, a low pressure-side constriction is provided between the one side surface and the columnar portion, a high pressure-side constriction is provided between the other side surface and the columnar portion, in a state where the piston is in contact with the slot, a top clearance volume (Vg) is formed by the high pressure-side constriction, the piston and the cylinder, a clearance volume (Vf) is formed by the low pressure-side constriction, the piston and the cylinder, and the top clearance volume (Vg) is smaller than the clearance volume (Vf).