A molecular beam source for use in thin-film accumulation, which enables the adjustment of the volume of a molecular beam, which is discharged per hour by using a needle valve, to be constant irrespective of a decrease in a thin-film element-forming material remaining within a crucible, contains heaters 32 and 42 for heating the thin-film element-forming materials “a” and “b” within crucibles 31 and 41, and valves 33 and 43 for adjusting the volumes to be discharged of molecules of the thin-film element forming materials “a” and “b”, which are generated within the crucibles 31 and 41. It further contains a controller for adjusting the opening of the valves 33 and 43 by servomotors 36 and 46 through feeding back information relating to the volumes of the molecular beams.