A mixer and device and method for manufacturing thin films are disclosed, comprising a mixer (1) capable of uniformly mixing gases with different weights, a thin-film manufacturing device with the mixer (1), and a thin-film manufacturing method using film forming gas formed by mixing the gases in the mixer, the mixer (1) comprising a mixing chamber (2) having the gas inlets (5a, 6a) of two gas inlet pipes (5, 6) positioned so as to be opposed to each other and a mixing gas diffusion chamber (3), wherein a partition plate (4) having a specific shape is installed between the mixing chamber and the diffusion chamber so that the volume of the diffusion chamber is larger than that of the mixing chamber, and a gas outlet (7) is provided in the partition plate at a specified position on the vertical lower side of a straight line connecting two gas inlets to each other. Manufacturing thin-film can be achieved by simple construction and low costs and occurrence of turbulent flow, convection, and thermal convection can be suppressed for uniformly forming gas flow of the mixed gases in the thin-film manufacturing device. In a vacuum reaction chamber (2), a stage (53) is arranged for carrying substrates. Opposite to the stage (53), a gas head (57) is mounted to a central portion of the top of the vacuum reaction chamber (2) to guide film forming gases into the vacuum reaction chamber. A cylindrical sleeve member (61) having a predetermined length is fitted to the side wall of the stage (53). The height of the stage is set at a position where the volume of a second space on the lower side of the stage connected to a vacuum exhaust means (55) is larger than that of a first space so that exhaust gas can be isotropically exhausted through a clearance between a sleeve member and the inside wall surface of the reaction chamber without causing convection in the first space between a gas head and the stage.