To provide a seal structure for a vehicular door, in which sealing property can be improved without having to increase the number of inner seal rubber or the like.

The inner seal rubber 10 of a door seal portion is branched at its tip 11 to allow the tip 11 to be widely expanded/narrowly contracted with an expanding/contracting means 12 arranged between the branched tip ends 11. A receiving member 19 having a groove 20 is provided at the opening portion frame 8 of a vehicle body. The branched tips 11 are inserted into the groove 20 of the receiving member 19 in a contracted state and are expanded in the groove 20 engaged with the groove, securely holding the seal rubber 10 to the opening portion frame 8 via the receiving member 19. Even when a door sash is externally expanded that is caused by the door being sucked out or the like due to the high speed traveling of the vehicle, seal between the door sash and the opening portion frame 8 with the rubber 10 can be favorably maintained without causing problems such as reduction in a contact area or occurrence of spaces, improving a sealing property at the door seal portion.