PURPOSE: To continuously produce a long panel comprising a thermoplastic composite by heating a core sheet to plastically deforming temp. heating the first contact surface of a skin sheet to the temp. lower than the process temp. of the core sheet and forming a large number of axially extending corrugations to the core sheet to bond one contact surface of the skin sheet to the core sheet.

CONSTITUTION: A first skin sheet 102 is advanced in an axial direction and the contact surface 103 thereof is heated to temp. lower than the temp. softening the sheet by an infrared heating means 22. At the same time, a core sheet 106 is advanced toward the first skin sheet to be heated to temp. making the surface of the core sheet sticky and capable of making the core sheet plastically deformable by infrared heaters 28a, 28b. Next, a large number of corrugations comprising upper and lower corrugation ridges 108, 110 axially extending as sets and extending axially so as to provide an interval are formed to a sheet. Next, pressure is applied to the region where the contact surface 103 of the first skin sheet 102 comes into contact with the sets of the lower corrugation ridges 110 of the core sheet 106.