PURPOSE: To solidify an adhesive applied to a position to be joined of a box efficiently at the same time with the high-frequency dielectric heating, by placing electrode plates connected to a source generating high frequency waves on two opposite faces of the box, and further placing auxiliary electrode plates on the other faces.

CONSTITUTION: In a box having one opened face in the six faces, and side plates 1, 2, 3 and 4 and a bottom plate 5 on the remaining five faces, electrode plates 6 and 7 connected to a high-frequency generating source are placed on the opened face and the bottom plate 5, and auxiliary electrode plates 8, 9, 10 and 11 (as the electrostatic capacity) unconnected to the high-frequency generating source are placed on the remaining side plates 1, 2, 3 and 4. A high-frequency electric power of 13.56MHz is applied across the electrode plates 6 and 7 to heat and solidify an adhesive applied to positions (a)W(h) to be joined. Gaps 12 and 13 of 20mm are preferably left between the auxiliary electrode plates 8, 9, 10 and 11 and each of the electrode plates 6 and 7.