To provide a heater unit which heats a casserole having a warm vegetable and can be attached to a part of tray to serve an in-flight meal inside an airplane.

A heat unit attached to the tray have a structure laminating a PTC heater 100 having positive temperature coefficient(PTC) and a planar heater 200. In the PTC heater 100, a PTC member 150 is buried inside an insulation base 130 and on the both sides of the PTC member, copper leaves 120, 122 as electrodes are piled up. The PTC heater 100 increases the electrical resistance value, stops heating and keeps the temperature when it reaches the settled temperature. The planar heater 200 contributes to shortening the time to raise the temperature right after starting heating.