A molecular beam source for use in accumulation of organic thin-films which enables the forming of a uniform thin-film on film-forming surfaces of a large-sized substrate, but without producing deposition or separation of a film-forming material in an opening for discharging molecules of the film-forming material, wherein a valve 33 is disposed in a space starting from a side of a molecule heating portion 12 and reaching to a molecule discharge opening 14 for discharging the generated molecules of the film-forming material towards a film-forming surface and, further, heaters 18 and 19 are provided at a side of the molecule discharge opening 14 for heating the molecules of the film-forming material to be discharged. At the side of the molecule discharge opening 14 are provided an exterior guide 13 having a tapered guide wall and also an interior guide 16 having a tapered guide wall is provided within an inside of the exterior guide. Between the exterior guide 13 and interior guide 16, there is formed a molecule discharge passage 17 in which the diameter thereof gradually increases along a direction of the discharge of the molecules therefrom. The heaters 18 and 19 are provided on the exterior guide 13 and the interior guide 16, respectively and, further, there is provided a heater 20 penetrating through the molecule discharge opening 14, whereby narrowing and/or blockage hardly occur in the discharge opening.