A sealed sleeve configured to be installed at an insertion coupling portion in a state in which a first pipe and a second pipe are insertion coupled to each other in order to seal couple the first pipe having a first outer diameter and a first inner diameter and the second pipe having a second outer diameter and a second inner diameter larger than the first outer diameter, the sealed sleeve comprising: a pillar shaped-main body having a main body central shaft; a through-hole formed using the main body central shaft as a central shaft on the main body and having an inlet as a second pipe side opening in an installation state and an outlet as a first pipe side opening in an installation state, wherein the through-hole has a pressing portion formed from a pressing portion second boundary portion positioned at a corner of the outlet to a pression portion first boundary portion existing at a predetermined position therein, configured to hermetically press the insertion coupling portion, wherein a length from the pressing portion first boundary portion to the pressing portion second boundary portion as a main body central shaft direction length of the pressing portion is formed in a length of 20 to 40% of a central shaft direction length of the main body, an inner diameter of the pressing portion is configured to be smaller than the second outer diameter of the second pipe and the pressing portion has the same inner diameter from the pressing portion second boundary portion positioned at a corner of the outlet to the pressing portion first boundary portion, the sealed sleeve is configured such that, in a mounting process of the sealed sleeve, as the sealed sleeve is gradually further inserted into the second pipe side, in order to gradually increase a pressure in which the sealed sleeve applies to the second pipe, the through-hole has a pressing introducing portion whose inner diameter gradually decreases over an area from the inlet to the pressing portion first boundary portion, an inner wall of the through-hole forms the pressing introducing portion whose inner diameter gradually decreases over an area from the inlet to the pressing portion first boundary portion, and a sectional outer edge line of the pressing introducing portion by a flat surface including the main body central shaft becomes a convex-shaped arc having a predetermined curvature radius. a pillar shaped-main body having a main body central shaft; a through-hole formed using the main body central shaft as a central shaft on the main body and having an inlet as a second pipe side opening in an installation state and an outlet as a first pipe side opening in an installation state, wherein the through-hole has a pressing portion formed from a pressing portion second boundary portion positioned at a corner of the outlet to a pression portion first boundary portion existing at a predetermined position therein, configured to hermetically press the insertion coupling portion, wherein a length from the pressing portion first boundary portion to the pressing portion second boundary portion as a main body central shaft direction length of the pressing portion is formed in a length of 20 to 40% of a central shaft direction length of the main body, an inner diameter of the pressing portion is configured to be smaller than the second outer diameter of the second pipe and the pressing portion has the same inner diameter from the pressing portion second boundary portion positioned at a corner of the outlet to the pressing portion first boundary portion, the sealed sleeve is configured such that, in a mounting process of the sealed sleeve, as the sealed sleeve is gradually further inserted into the second pipe side, in order to gradually increase a pressure in which the sealed sleeve applies to the second pipe, the through-hole has a pressing introducing portion whose inner diameter gradually decreases over an area from the inlet to the pressing portion first boundary portion, an inner wall of the through-hole forms the pressing introducing portion whose inner diameter gradually decreases over an area from the inlet to the pressing portion first boundary portion, and a sectional outer edge line of the pressing introducing portion by a flat surface including the main body central shaft becomes a convex-shaped arc having a predetermined curvature radius.