Here is disclosed a method for generation of a molecular beam from a sample solution, comprising steps of operating a spray-in device to introduce the sample solution in atomized state into a spray chamber, impinging a suitable gas on the sample solution in atomized state or heating the sample solution in atomized state to generate solute molecules deprived of solvent molecules, and then ejecting these solute molecules through an orifice into a low air pressure chamber. The apparatus as well as the method according to the present invention enable to generate a molecular beam for a variety of molecules, particularly for the neutral molecules which can be decomposed by heating at a high temperature or those which can not be sublimated or vaporized even by heating at a high temperature, as long as the sample solutions are prepared. Due to the method as well as the apparatus according to the invention, it is possible to conduct the mass spectroscopy studies and also other spectroscopic analyses about the molecules and the molecular clusters containing in the molecular beam generated in this manner, for example, by irradiating laser beams. It is possible to also possible to deposit the molecules on a substrate.