A population of the butterfly Luehdorfia japonica dependent on a small stand of an unusual host plant, Asarum caulescens, was found and observed for 5 years. The plant was growing in patchy clumps in a forest and in an adjoining clearing, but the number of leaves in the clearing decreased during the 5 year study period. Butterflies laid eggs mostly in or near the clearing, and the number of eggs steadily decreased year by year, along with the decrease in the number of leaves. The survival rate of the larvae was similar to that of the same species feeding on the usual host plant, Asarum takaoi, in an adjacent habitat. It is considered that A. caulescens is rarely a host of L. japonica because the plant grows in shady conditions and the butterfly prefers a host plant growing in a sunny locations for oviposition.