A heating device for heat-sealing bottom portions of containers (11) comprises a primary heater (41) and a secondary heater (42) arranged respectively at a first stop station and a second stop station of bottom forming radial mandrels, each of the mandrels (32) being adapted to carry a tubular blank (11) of rectangular to square cross section as fitted therearound with a bottom forming end portion thereof projecting outward from the mandrel. The primary heater (41) has a primary hot air nozzle. The secondary heater (42) has a secondary hot air nozzle. The primary hot air nozzle is fixedly provided on a phantom outward extension line of the mandrel as halted in the first stop station so as not to interfere with the end portion of the blank fitted around the halted mandrel and having orifices directed toward local parts h, i, j, k of the region of the end portion to be heated. The secondary hot air nozzle is movable on a phantom outward extension line of the mandrel as halted in the second stop station so as to advance into and retract from the end portion of the blank fitted around the mandrel as halted in the second stop station, and has orifices directed toward a plurality of portions uniformly distributed over the entire region a, b, c, d when the secondary hot air nozzle advances.