PURPOSE: An inexpensive antimicrobial composition that contains tetrachloroisophthalonitrile, a trihalogenated phenol and an o-phenylphenol, thus controlling microorganisms injuring industrial products in low concentrations.

CONSTITUTION: Tetrachloroisophthalonitrile, a trihalogenated phenol or its salt and an o-phenylphenol or its salt are used as effective components to give the objective antimicrobial composition. Coatings, wood or bamboo products, leather products, etc. are mixed or treated with the composition on the surface to inhibit the growth of injurious microorganisms and prevent the deterioration of their quality. Agents which show unsatisfactory effects, when they are used separately, are combined to expand the range of effective spectra against microorganisms and develop synergism, resulting in the low does application (less than 1/3W1/5 of the dose when they are used separately) to microorganisms which have been difficult to control by conventional methods. Especially, it shows excellent activity of controlling fungi belonging to Trichoderma, Aspergillus, etc. and bacteria belonging to Pseudomonas.