To provide an infiltrating method of a medicine, which develops no ununiform pouring irrespective of the kind of a tree with a simple method without necessitating a pouring device under vacuum and pressure.

In order to infiltrating a medicine in a wood, the wood, the water content of which is above its fiber saturation point, is immersed in an oily processing liquid, which does not mix with the water at an arbitrary ratio, under the temperature not lower than the room temperature and not higher than 100°C so as to infiltrate the medicine in the wood through the replacement of the water with the processing liquid. As the medicine to be used, any medicine, which is used generally and conventionally for a WPC, a modified wood, a preservation of wood against decay and insect, a dyeing or the like as long as one does not mix with the water at an arbitrary ratio and has oily properties, may well be used.