Heat shock treatment at 55℃ for 5 to 10 sec before inoculation induced the disease susceptibility of broad bean to B. cinerea. Such heat-induced susceptibility was manifested by lesion development and hyphal growth of pathogenicity-lost strains of B. cinerea in heatshocked broad bean leaflets. On the other hand, when virulent strain of B. cinerea was inoculated to the broad bean, appressorium formation and infection hyphal growth were also promoted in the dark. Under red light, however, infection behaviors were inhibited even in heat treated broad bean. Higher anifungal activity was observed in inoculation droplet recovered from heat-shocked broad bean leaflet kept under red light compared with that in the dark. These studies suggested that red light suppressed the heat-induced susceptibility in broad bean leaves inoculated with B. cinerea by accumulation of antifungal substance(s) in inoculation droplet.