A plasma processing apparatus includes a chamber having a sidewall and a plasma processing space surrounded by the sidewall, and a first side gas inlet line and a second side gas inlet line configured to introduce at least one gas from the sidewall into the plasma processing space. The first side gas inlet line includes a plurality of first side gas injectors symmetrically arranged along a circumferential direction on the sidewall and configured to introduce the gas in a first direction into the plasma processing space. Further, the second side gas inlet line includes a plurality of second side gas injectors symmetrically arranged along the circumferential direction on the sidewall and configured to introduce the gas in a second direction different from the first direction into the plasma processing space.