Wettability is a phenomenon where the contact angle of liquid against solid surface is kept, and representing wettability is one of the most important issues in physical simulations. All previous works using particle methods specified the contact angle and rectified the behavior of particles constructing the droplet. However, wettability occurs not by the specification of the contact angle but by the interaction between solid and liquid based on the physical properties. Therefore, the authors propose a new method to represent wettability not by specifying the contact angle but by considering adhesional and spreading wettings. The proposed method uses the average pressure in the fluid-structure interaction analysis for stable analysis even in the cases where the number of particles in the droplet is small or the contact surface region between the droplet and the structure is small. As the result of the simulations, the contact angle reproduced by the proposed method was more accurate than that by the previous method without specifying the contact angle. In addition, the phenomenon of a droplet sliding down on a leaf surface maintaining the high contact angle has also been represented by the fluid-structure interaction analysis.