An apparatus for mounting a siphon tapping tube onto a tap hole mounting part of a converter vessel has a lower car and an upper car, which are placed on a ladle car which travels beneath a converter vessel, the lower car and the upper car being capable of travelling in the direction of the travel of the ladle car and in the direction perpendicular to that of the travel of the ladle car, respectively, a loading table mounted on the upper car, which is guided by an upright support frame and being capable of moving upward and downward with a siphon tapping tube loaded on the table, and bolt tightening and untightening units which are provided on the loading table in positions corresponding to the respective bolt hole set positions in the flange of the siphon tapping tube and are capable of receiving mounting bolts. TV camera is provided on the upper car, allowing the operator to monitor the tap hole mounting part.