PURPOSE: To provide a steel pipe for use as a car door reinforcing member which reduces the weight of the door without damaging its strength characteristic.

CONSTITUTION: A steel pipe 1 (for use with length L) suspended in a space between a door outer plate and a door inner plate and being uniform in outside diameter and in wall thickness largest at its center portion (A portion) and smallest at the middle portion (B portion) between its end portion and its center portion, with the end portion (C portion) of the steel pipe 1 having an intermediate thickness between the thickness of the center portion and that of the middle portion. The pipe may be uniform in wall thickness and may in outside diameter be largest at the A portion, smallest at the B portion and intermediate at the C portion. Alternatively, both the wall thickness and the outside diameter of the pipe may be varied as mentioned above.