Novel imidazole derivatives and their acid adducts are disclosed. The imidazole derivatives are represented by formula: ##STR1## wherein R.sub.1 represents an alkyl group, R.sub.2 represents a hydrogen atom, an alkyl group which may have a substituent, or an aralkyl group which may have a substituent, provided that R.sub.1 and R.sub.2 are not both methyl groups at the same time. They have strong antimicrobial activity, especially against those fungi belonging to the genera Candida, Trichophyton, Microsporum, and Epidermorphyton, as well as against Gram-positive bacteria, and thus is useful as an antimicrobial agent.