A sealing mechanism of an automobile door including a sliding window pane configured to be raised or lowered to/from a door window part and a side panel fixed to the door window part, the sealing mechanism comprises: a retainer fixed to the side panel; and a glass run held by the retainer, wherein the glass run includes a hollow seal part including a contact part to be in contact with an interior side of the sliding window pane and a base provided at a back of the contact part, the base of the hollow seal part has a slit extending along a longitudinal direction of the glass run, the retainer includes a retainer body (22, 23) extending along a back of the base, a pair of engagement parts (22a, 22b) provided in the retainer body and engaged with both sides of the base, and a ridge projecting toward an exterior relative to the retainer body, located between the engagement parts (22a, 22b) and configured to be fitted in the slit of the base, the pair of engagement parts (22a, 22b) engaged with both sides of the base and the ridge fitted in the slit allow the retainer to hold the base, and the glass run and the ridge of the retainer constitute a hollow portion in a closed cross section extending along a longitudinal direction of the glass, characterized in, that the glass run is an integrated unit of a glass-run upper part extending upward from a position lower than a belt line of the door and a glass-run lower part extending downward from the position lower than the belt line, the glass-run upper part includes the hollow seal part including the base having the slit, the glass-run lower part has a channel shape, the ridge of the retainer is provided at least in the glass-run upper part, and in the glass-run upper part, the ridge of the retainer is fitted in the slit of the base, and a center of the base of the glass-run upper part protrudes toward an interior relative to an interior surface of the glass-run lower part to form a protruding portion, and the slit in which the ridge is fitted is provided in the protruding portion of the base. a retainer fixed to the side panel; and a glass run held by the retainer, wherein the glass run includes a hollow seal part including a contact part to be in contact with an interior side of the sliding window pane and a base provided at a back of the contact part, the base of the hollow seal part has a slit extending along a longitudinal direction of the glass run, the retainer includes a retainer body (22, 23) extending along a back of the base, a pair of engagement parts (22a, 22b) provided in the retainer body and engaged with both sides of the base, and a ridge projecting toward an exterior relative to the retainer body, located between the engagement parts (22a, 22b) and configured to be fitted in the slit of the base, the pair of engagement parts (22a, 22b) engaged with both sides of the base and the ridge fitted in the slit allow the retainer to hold the base, and the glass run and the ridge of the retainer constitute a hollow portion in a closed cross section extending along a longitudinal direction of the glass, characterized in, that the glass run is an integrated unit of a glass-run upper part extending upward from a position lower than a belt line of the door and a glass-run lower part extending downward from the position lower than the belt line, the glass-run upper part includes the hollow seal part including the base having the slit, the glass-run lower part has a channel shape, the ridge of the retainer is provided at least in the glass-run upper part, and in the glass-run upper part, the ridge of the retainer is fitted in the slit of the base, and a center of the base of the glass-run upper part protrudes toward an interior relative to an interior surface of the glass-run lower part to form a protruding portion, and the slit in which the ridge is fitted is provided in the protruding portion of the base.