An inductively coupled plasma reactor comprising: a reactor body having a plurality of plasma discharge chambers positioned in multistage arrangement and in parallel, the reactor body having gas entrances with a number of holes opened towards the upper plasma discharge chambers positioned at an upper position of the plasma reactor; a transformer including a magnetic core and a primary winding, the magnetic core being shared between at least two plasma discharge chambers, and being positioned across each one of the plasma discharge chambers, parts of the magnetic core being mounted so as to be externally exposed out of the reactor body; a core protecting tube arranged to cover and protect parts of the magnetic core positioned inside each one of the plasma discharge chambers; and a power supply source connected to the primary winding, and wherein a current of the primary winding is driven by the power supply source, the driving current of the primary winding inducing AC potential which generates inductively coupled plasma to complete a secondary circuit of the transformer, and the inductively coupled plasma being generated in each one of the plurality of plasma discharge chambers to cover the outside of the core protecting tube, and wherein the reactor body further comprises multiple gas exits opened downward in the lower plasma discharge chambers positioned at a lower position of the plasma reactor, the reactor body further having connection paths with a number of holes which connect the plasma discharge chambers at the upper and lower positions to each other, whereby a gas flow path passing through a number of the plasma discharge chambers is formed between the gas entrances and the gas exits. a reactor body having a plurality of plasma discharge chambers positioned in multistage arrangement and in parallel, the reactor body having gas entrances with a number of holes opened towards the upper plasma discharge chambers positioned at an upper position of the plasma reactor; a transformer including a magnetic core and a primary winding, the magnetic core being shared between at least two plasma discharge chambers, and being positioned across each one of the plasma discharge chambers, parts of the magnetic core being mounted so as to be externally exposed out of the reactor body; a core protecting tube arranged to cover and protect parts of the magnetic core positioned inside each one of the plasma discharge chambers; and a power supply source connected to the primary winding, and wherein a current of the primary winding is driven by the power supply source, the driving current of the primary winding inducing AC potential which generates inductively coupled plasma to complete a secondary circuit of the transformer, and the inductively coupled plasma being generated in each one of the plurality of plasma discharge chambers to cover the outside of the core protecting tube, and wherein the reactor body further comprises multiple gas exits opened downward in the lower plasma discharge chambers positioned at a lower position of the plasma reactor, the reactor body further having connection paths with a number of holes which connect the plasma discharge chambers at the upper and lower positions to each other, whereby a gas flow path passing through a number of the plasma discharge chambers is formed between the gas entrances and the gas exits.