A seat belt mechanism comprising a first through member attached to a sash of a door, a second through member supported in the door, a shoulder belt inserted through the first through member and a waist belt inserted through the second through member, respective one ends of the belts being reeled into a retractor, while respective other ends of the belts being held by a belt hold member fixed to a vehicle body or a seat. The second through member is moved upwardly and downwardly in a guide provided in the door by an actuator device depending on opening the closing of the door, so that a passenger can easily get into out of the vehicle without having unpleasant feelings.