A metal pipe inner surface weld-mounting apparatus is constituted by an arm (3), a horizontal torch (4), a mirror (21; 80), and a camera (24). The arm (3) has a supported base end portion, which is horizontally extended. The horizontal torch (4) is attached to a leading end portion of the arm. The weld-mounting is performed on an inner surface portion of a metal pipe (w) supported rotatably around a horizontally extending axis (14) while at least one of the metal pipe (w) and the horizontal torch (4) is moved in a direction of the axis. The mirror (21; 80) is attached to the leading end portion of the arm. The camera (24), is disposed in a position separated from the mirror for picking up an image (25), of weld-mounting beads (23), in a neighbourhood of arc light (22), of the horizontal torch (4) through the mirror (21; 80). Preferably, a monitoring device (26; 70) monitors a condition of the weld-mounting.