To avoid dangers by allowing the driver of a vehicle to check blind spots on the sides of the vehicle as much as he can.

This concave mirror band and a device for mounting the same on a rear-view mirror are so designed that a bolt 7 is passed through a spring 8 and a horizontal adjustment frame 3 and is tightened against a mounting stay 4 for the rear-view mirror and the concave mirror band 1 is bonded to a vertical adjustment frame 2 and fastened to the horizontal adjustment frame 3 by screws 5. Therefore, blind spots on the sides of a vehicle, which could not be checked with rear-view mirrors of the prior art, can be checked as much as possible.