It has been known amoung specialists that there is a monophyllous pine at Koigakubo, Kokubunji-machi, Tokyo-to. The results of anatomical study of its leaves are as follows. 1. The leaves have a little more resin canals than ordinary leaves. 2. The position of resin canals is inside mesophyll type. (Pinus Thunbergii type) 3. The number of vascular bundles is two. 4. The cross section of leaves is nearly round, but no adherence of two needles is perceptible. From the above facts, it is supposed that in the case of these leaves one is abnormally developed and the other completely degenerated.