A base material is placed on a base material placement face of a base material placement table. An inductively coupled plasma torch unit is structured with a cylindrical chamber structured with a cylinder made of an insulating material and provided with a rectangular slit-like plasma jet port, and lids closing opposing ends of the cylinder, a gas jet port that supplies gas into the cylindrical chamber, and a solenoid coil that generates a high frequency electromagnetic field in the cylindrical chamber. By a high frequency power supply supplying a high frequency power to the solenoid coil, plasma is generated in the cylindrical chamber, and the plasma is emitted from the plasma jet port to the base material. While relatively shifting the plasma torch unit and the base material placement table, a base material surface can be subjected to heat treatment.