PURPOSE: To pyrolytically cracking a hydrocarbon in a cost-effective manner by heating a hydrocarbon vapor feedstock material not satd. with water vapor and water by indirect heat exchange to evaporate a part of water, conducting cracking in a pyrolysis oven in the presence of evaporated water to prepare an olefin.

CONSTITUTION: A hydrocarbon vapor feedstock not satd. with water vapor is introduced through a conduit 12. The feedstock is preheated with a preheating coil 14, fed into an exhaust heat utilization tank 24 and heated together with water fed from a water distributor 30 by indirect heat exchange to evaporate at least a part of water. The residue is combined with a hydrocarbon vapor feedstock, and water remaining unevaporated is separated from the hydrocarbon vapor feedstock and evaporated water. A hydrocarbon vapor feedstock is introduced through a conduit 38 into a cracking oven 18 in the presence of evaporated water. Cracking is carried out in a cracking section 54 to prepare a cracking oven discharge stream comprising an olefin hydrocarbon product gas and evaporated water, and the discharge steam is cooled in a rapid cooling tank 58. Thus, a hydrocarbon vapor feedstock is pyrolytically cracked for the preparation of an olefin hydrocarbon.