A method of using a film formation apparatus for a semiconductor process includes removing by a cleaning gas a by-product film deposited on an inner surface of a reaction chamber of the film formation apparatus, and then chemically planarizing the inner surface of the reaction chamber by a planarizing gas. The inner surface contains as a main component quartz or silicon carbide. The removing is performed while supplying the cleaning gas into the reaction chamber, and setting the reaction chamber at a first temperature and first pressure to activate the cleaning gas. The planarizing is performed while supplying the planarizing gas into the reaction chamber, and setting the reaction chamber at a second temperature and second pressure to activate the planarizing gas. The planarizing gas contains fluorine and hydrogen fluoride. The second temperature is within a range of from 300 to 800° C.