A seatbelt system for a front seat of a vehicle having such a double door that a front door (2) and a rear door (3) open outwardly forward and rearward, respectively, to form an uninterrupted doorway (14) to a passenger compartment (A) and the front door (2) overlaps the rear door from the outside so as to allow the rear door (3) to open and close while the front door is open comprises a retractor (28) installed in the rear door (3) and retracting a seatbelt (27) therein, a seatbelt lock mechanism (37) operative to lock the seatbelt (27) when the seatbelt (27) attains a predetermined acceleration, and a lock canceling mechanism (38) operative to put the seatbelt lock mechanism (37) inoperative when activated, The seatbelt system activates the lock canceling mechanism (38) to put the seatbelt lock mechanism (37) inoperative when the seatbelt (27) is unbuckled while the front door and/or the rear door is open or not properly closed and deactivates the lock canceling mechanism (38) to put the seatbelt lock mechanism (37) operative when the seatbelt (27) is buckled up while the front door and/or the rear door is open or not properly closed.