To provide a plasma processing apparatus easy to be installed in a manufacturing facilities without disturbance in plasma and without causing a large size of the apparatus.

The plasma processing apparatus has a tool to generate a plasma from a gas by introducing the gas into a vacuum vessel 4 with a window 13 closed with a disk-shaped dielectric substance 3 and by applying high frequency electric power via the dielectric substance 3, and is installed with a surface wave detection antenna 5 to detect a signal of the surface wave 10 propagating through the dielectric substance 3 and an oscilloscope 8 to detect electric field intensity distribution by receiving the signal of the surface wave 10 and by measuring signal strength equivalent to the electric field intensity and is provided with a density measuring tool to determine the plasma density by the detection of the electric field intensity distribution.