A vehicular seat belt device including: a belt reel on which a webbing that restrains an occupant is wound; an actuator that generates instantaneous rotational drive force by gas that is generated during an emergency; a flywheel that rotates by the rotational drive force of the actuator; a speed increasing mechanism that is housed inside the flywheel and that increases the rotational speed of the actuator and transmits it to the flywheel; and a connecting and disconnecting unit that connects a low-speed rotation side of the speed increasing mechanism and the belt reel during rotation of the flywheel, wherein the belt reel rotates to the webbing winding-up direction by the rotational drive force of the actuator.