PURPOSE: To easily obtain a carrier, especially, for a nucleotide reagent or condensing agent for the synthesis of nucleic acid, by using a specific preparing intrument obtained by freeze drying a porous substrate impregnated with a predetermined amount of a solid reagent solution in predetermined concn. to make the dried impregnated substrate preservable in a hermetically closed container.

CONSTITUTION: A columnar body 35 comprising a porous filter substrate made of polyethylene, polypropylene, ceramics, glass or cotton is put in a hermetically closable tool 30 used in preparing a reagent supported porous substrate comprising a container 34 consisting of an upper lid 31, a container main body 32 and a lower lid 33 and the main body 32 thereof is immersed in a solution of a nucleotide reagent or condensing agent for DNA or RNA synthesis while a syringe S is used to impregnate the porous substrate inserted into the main body with the solution L. In the next step, the lower lid 33 is applied to the main body 32 to freeze the solution L in a cryogen and, after freeze-drying, the upper lid 31 is applied to the main body 32 to make the reagent stably preservable. In use, the upper and the lower lids are detached to add a solvent and the definite amount of the reagent can be supplied to a reactor.