We investigated the growth inhibition effect of pyroligneous acid on the pathogenic fungus, Alternaria mali, which is known to be the agent of Alternaria blotch of apple plants. Chemical control of A. mali could be achieved through the use of agrochemical fungicides, while the substitute for agrochemical control is gradually increasing. It was observed that pyroligneous acid exhibited antifungal activity against some plant pathogenic fungi. More specifically, the growth of A. mali was completely inhibited in pyroligneous acid at a dilution of 1:32. When its antifungal activity was compared to that of polyoxin S, which is used for the chemical control of Alternaria blotch of apple, it was observed that the antifungal activity of pyroligneous acid diluted at 1:32 corresponded to 2.0mg/mL of polyoxin B. Consequently, it is concluded that the diluted pyroligneous acid can substitute for polyoxin B, thereby reducing the use of the agrochemical for the control of Alternaria blotch of apple.