In an atmospheric inductively coupled plasma generating apparatus, impedance matching between a coil for plasma generation and an RF power source is effected at a high speed. A control method and/or condition for the output frequency of an oscillator which supplies a power to the coil for plasma generation or a control method and a control condition for the output power of the oscillator are changed appropriately in accordance with a generation state of a plasma. When a plasma is not generated, the output frequency is controlled according to a first condition and, when a plasma is generated, the output frequency is controlled according to a second condition different from the first condition. When a plasma is not generated, the output power is controlled according to a third condition and, when a plasma is generated, the output power is controlled according to a fourth condition different from the third condition.