The present invention provides a method capable of easily mixing any liquid containing a linking substance such as a divalent metal cation and the like with a liquid containing a particular compound at a high concentration, and capable of producing a liquid medium composition comprising fine structures dispersed therein, and a production device therefor and a kit therefor. The first liquid containing a particular compound is passed through a through-hole having a given cross-sectional area formed in a nozzle part at a given flow rate and injected into the second liquid at a given flow rate. By this simple operation, a structure in which the particular compound is bonded via the linking substance is formed, and the structure is preferably dispersed in a mixture of the both liquids.