We have been developing a high efficiency and compact induction thermal plasma torch using a static induction transistor inverter in the frequency range of 0.5∼1.5MHz. The ignition of an initial plasma is assisted by spark discharges using an automobile spark plug. The use of spark discharges makes the ignition of the induction plasma reproducible and stable. The threshold rf current of the induction coil required for start-ups is strongly reduced to be about half of that without spark discharges. The rf voltage and current measurement in the rf inverter circuit shows that more than 80% of the generated rf power is consumed to sustain argon thermal plasmas at the atmospheric pressure.