A seatbelt mechanism (S) comprises a chip anchor (12) disposed to one of right and left sides of a seat bottom (3) of the vehicle seat (1) and coupled to a tip end of the seatbelt (10), and a tongue anchor (14) disposed to the other of the right and left sides of the seat bottom (3) and engaged with a tongue plate (13) disposed to an intermediate portion of the seatbelt (10). A pretensioner mechanism (21) moves both the chip anchor (12) and the tongue anchor (14) substantially at the same time to actively tenses the seatbelt (10) when the vehicle collision occurs. A moving distance of the chip anchor (12) is substantially equal to a moving distance of the tongue anchor (14).