PURPOSE: To enhance a conversion ratio of a heavy asphaltene-containing hydrocarbon feedstock to a product having a lower b.p. by pyrolyzing the raw material under a specific condition.

CONSTITUTION: A hydrocarbon feedstock containing heavy asphaltene composed of 25 wt.% or more of a hydrocarbon with a b.p. of 520°C or higher pref. being a vacuum residual oil of crude oil is preheated to be passed through a pyrolyzing zone held to a condition setting a conversion ratio of hydrocarbon with a b.p. of 520°C or higher to 35-70 wt.% and, subsequently, the effluence from the pyrolyzing zone is separated into one or more kinds of a distillation fraction (e.g.; gas, gasoline, intermediate fraction, vacuum distillate) and a residual fraction, and the residual fraction is deasphalted to be separated into asphalt and deasphalted oil.