A structure and method for mounting a flag-type side mirror securely fastens the flag-type side mirror on a door panel of a vehicle. The method includes: passing a wiring mounted to the flag-type side mirror through through-holes formed in an outer belt rail, a quadrant reinforce, and an inner panel of the door; placing the flag-type side mirror at a position to be installed; arranging a mirror neck of the flag-type side mirror to abut on the quadrant reinforce through an outer panel of the door; passing a fastening member through the outer belt rail and the quadrant reinforce; and fastening the fastening member to an end of the mirror neck by operating a tool in a direction from inside to outside of the vehicle.