PROBLEM TO BE SOLVED: To provide a heating method of a fiber containing resin body which improves heating efficiency of the fiber containing resin body without affecting the orientation of a reinforcement fiber in the fiber containing resin body, and to provide a heater of the fiber containing resin body.SOLUTION: A heater 10 of a sheet body 1 includes an infrared heater which heats the sheet body 1 for melting the sheet body 1 and further includes an infrared ray transmission plate 25 which restricts the sheet body 1 from expanding in a thickness direction during heating and holds a shape of the sheet body 1.