A disinfectant which contains olanexidine in such a concentration that the disinfectant shows an effective bactericidal activity and which is reduced in side effects such as skin irritation. The disinfectant comprises an aqueous solution which contains olanexidine and gluconic acid, the amount of the latter being at least equimolar with the former, and contains substantially none of acids other than gluconic acid and of salts of the acids (other than gluconic acid).