A plasma generating apparatus and a method for controlling plasma-generating high frequency power capable of appropriately and easily controlling plasma density over the long term are disclosed. Measuring high frequency power is supplied to plasma through a dielectric tube. Apart from a supply system of plasma-generating high frequency power, there is provided a plasma density information obtaining section having long lifetime no hot filament. The plasma density information obtaining section obtains the plasma density information by measuring a physical amount related to reflection or absorption of the high frequency power. An actually measured plasma density monitored by the plasma density information obtaining section from moment to moment and a target plasma density held in a plasma density setting section are compared. Based on this compared result, an impedance matching device is changed, and an impedance matching state between the electric source side and the plasma side, thereby appropriately controlling the generating high frequency power output from the high frequency electric source.