Sealing mechanism of automobile door

A sealing mechanism of an automobile door including a sliding window pane configured to be raised or lowered to/from the door window part and a side panel, includes: a retainer fixed to the side panel; and a glass run held by the retainer. The glass run includes a hollow seal part, and the base of the seal part has a slit. A pair of engagement parts of the retainer is engaged with both sides of the base of the seal part. A ridge located between the engagement parts of the retainer is fitted in the slit of the base.