PURPOSE: To obviate defective joining at joined sections when a character sheet is distended into a top force recessed sections by a high frequency electric current to form a pattern even if a character sheet and a stock sheet the difference of the dielectric losses of which are great, by interposing previously a melted resin between the character sheet and the stock sheet.
CONSTITUTION: A melted resin 3 such as polyurethane type resin or the like is interposed by coating between a stock sheet 1 of a nylon type or the like and a character sheet 2 of PVC or the like. The sheets 1, 2 are pressed between a top force 4 having recessed sections 5 and a bottom force and a high frequency electric current is passed between the electrodes at the joined sections 6. Due to the dielectric loss and the heat generated between the sheets 1, 2 the resin 3 melts thereby thermally joining them at joining sections 6. At the same time, due to the action of the air thermally expanded between the sheets 1, 2, the sheet 2 is plastically deformed into the recessed sections 5 of the top force 4 so that three- dimensionally bulged characters and patterns may be formed.
EFFECT: If the method is applied to form a bag that uses a nylon type fabric, the method can be carried out in a short period with high bonding strength resulted.
