To provide a device for decomposing an org. halogen compd. capable of accelerating neutralizing reaction by micronizing acidic gas bubbles blown into a waste gas treating tank.

In the decomposing device, thermal plasma is produced by applying microwave to the gas containing the org. halogen compd. to decompose the org. halogen compd. in the thermal plasma and the product gas produced by decomposition reaction is released into alkali soln. as bubbles K1 from a blowing pipe 45 to neutralize. The contact surface with an alkali soln. is increased and also the time required for reaching the bubbles to liq. surface is lengthened and neutralizing reaction is accelerated since the product gas released from the tip of the blowing pipe 45 as the bubbles K1 are divided to pieces by colliding to a turbine type impeller T of a bubble dividing means 52 for accelerating the neutralizing reaction by dividing the bubbles K1.