The present invention provides an assembling apparatus for a semiconductor manufacturing apparatus. The assembling apparatus includes: a body; lift attached to the body and configured to move a reaction tube having an opening at a lower end portion thereof vertically, thereby allowing a gas supply pipe to be installed inside the reaction tube through the opening while the reaction tube is held bythe lift; gas supply source configured to supply a gas into the reaction tube through the gas supply pipe while the reaction tube is held by the lift; and an exhaust mechanism including a pump configured to exhaust an inside of the reaction tube through the opening, thereby performing a leakage test of the reaction tube while the reaction tube is held by the lift.