PURPOSE: To produce isobutylene in a high purity and in a high yield by subjecting t-butylalcohol to a gas phase dehydration reaction in the presence of a catalyst prepared by carrying an aluminum compound decomposable at ≤a certain temperature on silica and subsequently heating and calcining the carried aluminum compound at ≥ the decomposition temperature thereof.

CONSTITUTION: When t-butylalcohol is subjected to a dehydration reaction in a gaseous phase in the presence of a catalyst to produce isobutylene, a catalyst being prepared by carrying an aluminum compound (e.g., aluminum sulfate or aluminum nitrite) decomposable at ≤1500°C on silica and subsequently heating and calcining the carried aluminum compound at the decomposable temperature (from the decomposable temperature to 1500°C) is employed for the reaction. The catalyst is industrially advantageous because of having a long catalyst life.