A pretensioner mechanism comprising webbing capable of restraining a vehicle occupant; a cylinder member into which gas is supplied when specific conditions occur; a movement member provided so as to be capable of moving inside the cylinder member, the movement member being formed with a communication hole that communicates a first side and a second side of the movement member; a closure member that is inserted into the communication hole and substantially closes off the communication hole to such an extent that pressure from the gas supplied into the cylinder member causes the movement member to move towards the first side and the restraining force of the webbing on the occupant is increased; and a communication groove formed between the communication hole and the closure member to communicate a first side of the closure member with a second side of the closure member.