To provide an inside beveling machine for a ring externally fitted on a steel pipe which can easily and efficiently bevel a round hole of the ring externally fitted on the steel pipe such as a stiffener ring, after completion of both polygonal periphery forming and round hole opening on a plate at reduced costs.

The inside beveling machine for the ring externally fitted on the steel pipe comprises a pair of left and right drive rollers 13, 14 for holding and rotating the ring 12 attached to a support 11 of the machine body 10 at its inner circumference, a guide roller 15 contacting against the bottom of the inner circumference, a guide means 16 for positioning the ring in its rotary direction, and a cutting means 17 cutting both sided peripheries on the inner circumference of the ring attached to the support.