PURPOSE: To prevent the lowering of the activity of a plazma initiator by maintaining the weight ratio of carbon to hydrogen within a specific range by the addition of molecular hydrogen, when hydrocarbon is modified by using microwave in radiation in a reaction zone including the plamza initiator.

CONSTITUTION: When a hydrocarbon supply material is modified by the irradiation of microwave radiation (preferably, that having at least l0.3 GHz of frequency) in a reaction zone including a plazma initiator comprising metal such as W, Fe or the like, non-metal such as carbon, alumina or the like, or a complex thereof, molecular hydrogen is added to the reaction zone, and the weight ratio of carbon in the molecular in contact with the plazma initiator to hydrogen is maintained at less than 6:1, particularly less than 1.5:1, whereby lowering in the activity of the plazma initiator can be prevented.