PROBLEM TO BE SOLVED: To solve such problems that an agent using a carboxylic acid metal salt and a pyrethroid insecticidal and/or germicidal component has insufficient formulation stability and that a water dilution of the agent to be used in an immersion treatment for an industrial purpose has insufficient long-term stability.SOLUTION: Formulation or a water dilution of a biocide composition with long-term stability can be obtained by incorporating a carboxylic acid metal salt, a pyrethroid insecticidal and/or germicidal component, a solvent and a surfactant. The carboxylic acid metal salt is preferably a carboxylic acid metal salt synthesized from a monovalent carboxylic acid. The solvent is preferably at least one solvent selected from esters, ethers, and aromatic hydrocarbons. The surfactant preferably comprises a polyoxyethylene-hardened castor oil ether or polyoxyethylene castor oil ether, and at least one selected from a nonionic surfactant and an anionic surfactant.
