A substrate processing apparatus that can appropriately carry out desired plasma processing on a substrate. The substrate is accommodated in an accommodating chamber. An ion trap partitions the accommodating chamber into a plasma producing chamber and a substrate processing chamber. High-frequency antennas are disposed in the plasma producing chamber. A process gas is introduced into the plasma producing chamber. The substrate is mounted on a mounting stage disposed in the substrate processing chamber, and a bias voltage is applied to the mounting stage. The ion trap has grounded conductors and insulating materials covering surfaces of the conductors.