Farmers' fields were surveyed for a new disease on broad bean in different parts of Shimane Prefecture. The disease was found in all surveyed fields. The fungus frequently associated with diseased broad bean leaves having the characteristic leaf spot symptom was identified as Alternaria tenuissima, based on morphological characteristics. The initial lesion was brown, water soaked, circular to slightly irregular. Then the lesion enlarged and became concentric. Mature leaves had coalescing necrosis surrounded by yellowing. Older leaves of the plant were particularly affected. In a later stage of the disease, the leaves became blighted from the margin to the center and most of the diseased plants defoliated. In severely infected fields, circular to irregularly shaped lesions also appeared on stems and pods, and plants defoliated completely. All tested isolates were pathogenic to broad bean leaves. Different isolates of the fungus varied in pathogenicity on different broad bean cultivars. In a comparison of symptoms of the disease and morphological characteristics of the isolated fungus with other broad bean diseases, this disease represented a new, as yet undescribed disease of broad bean in Japan. Therefore, we propose the name "leaf spot of broad bean" and "Soramame hanten-byô" in Japanese.