A thin film-forming apparatus, for ensuring uniform plane distribution of properties of a film formed on a substrate surface, has a gas-supply port 24a supplying a gas mixture from a gas-mixing chamber 24 to a shower head 25. The port is arranged at the peripheral portion on the bottom face of the gas-mixing chamber so that the gas mixture flows from the upper peripheral region of the head towards the center thereof. An exhaust port 32 discharging the exhaust gas generated in the film-forming chamber 3 is arranged at a position lower than the level of a stage 31 during film-formation directing the exhaust gas towards the side wall of the chamber 3 and discharging the exhaust gas through the exhaust port. The stage 31 is designed to move freely up and down to adjust the distance between the shower head 25 and substrate S.