a safety belt wind - up device includes a rotatable belt wind - up shaft , wind - up force lock device capable of assuming interlocked and uninterlocked positions with the wind - up shaft including a first member rotatively driven by the wind - up shaft when the wind - up force lock device is in the interlocked position , a first cam rotating together with the first member , a second member rotatively driven by the first member engaging the second member , and a second cam rotating together with the second member , and a device for bringing the lock device into the interlocked and uninterlocked positions depending upon engagement and disengagement of a buckle of the device . the device permits a safety belt to be drawn out of the device to a length required for a user &# 39 ; s action from the equipped position and completely delete the wind - up force acting upon the belt at the equipped position , whereby the user is not subjected to a belt wind - up force .