PURPOSE: To obtain a stable aqueous solution of vitamin K2 storable without causing clouding for a long period, and useful especially as an injection, by dissolving vitamin K2 in water in the presence of refined lanolin and a synthetic solubilization agent.

CONSTITUTION: A stable aqueous solution of vitamin K2 of formula (general name: menatetrenone) can be produced by adding a mixture of a synthetic solubilization agent and refined lanolin (a substance harmless to human body) to vitamin K2 (a fat-soluble vitamin). The synthetic solubilization agent is e.g. a nonionic surfactant such as polyoxyethylene hardened castor oil, sorbitan fatty acid ester, etc., or a solvent such as propylene glycol, etc. The refined lanolin is a substance registered in Japanese Pharmacopeia, allowed to be administered continuously to human body as a base component of ointment or cream, and having a chemical component consisting of a fat-like substance obtained from wool, i.e. was. The amount of the refined lanolin is preferably 20W40wt% of the vitamin K2.