PURPOSE: To shorten a baking curing time without generating baking blister at the time of the baking of the coating applied to the inner surface of a pipe in a hot air oven.

CONSTITUTION: Paint is heated to temp. of 60-90°C within a short time by preparatory treatment using electromagnetic induction heating before the viscosity of the coating is excessively increased and the volatilizing speed of a volatile substance increases suddenly and subseuently raised in temp. gradually by hot air to be cured. The baking of the coating suppresses productivity in a pipe inner surface coating process but a baking time is shortened to about 3/4 and productivity is enhanced.