Provided is a production method for producing a solid product by a reaction of gaseous raw materials with a plurality of components including a step of conducting the reaction using a reactor disposed in a vertical direction; a step of feeding the gaseous raw materials with a plurality of components from the upper part of the reactor; a step of, in the lower part of the reactor, forming a seal gas layer composed of a gas having a high density and fed continuously from the lower part of the reactor; a step of discharging an exhaust gas containing a by-product gas generated by the reaction and unreacted gaseous raw materials from somewhere in the upper part of the formed seal gas layer; and a step of accommodating a solid product in the seal gas layer of the lower part.