A seat belt anchor device according to one aspect includes: an anchor plate including a loop attached portion for a seat belt webbing, and having an opening including an enlarged region into which a head of a stud is allowed to be inserted and a lock region in which the head of the stud is not allowed to enter from the enlarged region but a shank of the stud is allowed to enter; a protector attached to the anchor plate and including a bridge portion disposed over the enlarged region on a side opposite to a vehicle body with respect to the enlarged region so that the loop is held between the protector and the loop attached portion; and a stopper attached to the anchor plate and configured to engage with the head of the stud when the shank of the stud enters the lock region from the enlarged region.