The present invention provides ammonia purification means to remove impurities such as oil with a negative effect on vapor deposition from inexpensive commercially available industrial crude ammonia and from crude ammonia recovered from the gallium nitride compound semiconductor process and to continuously supply the purified ammonia to the gallium nitride compound semiconductor process. The oil removing device removing oil from crude ammonia containing oil as impurities includes an oil filter cylinder accommodating a filtration element cylindrically formed from a filtration membrane processed into a shape of pleat, honeycomb, or space structure; and an oil adsorption cylinder filled with activated carbon. The ammonia purification apparatus is provided with the oil removing device; a catalyst cylinder filled with a catalyst containing nickel as an active component; and an adsorption cylinder filled with synthetic zeolite.