In this paper, we propose an improvement in boundary condition computation of Smoothed Particle Hydrodynamics. Generally, to calculate a wall boundary in particle methods, a wall is converted into particle representation and used as static particles. However, it has several issues, e.g., a boundary is discretized into particles and thus does not result in accurate representation of the boundary shape. For precise representation of the shape, the resolution of a simulation must be increased and thus a large number of particles are required. The proposed method does not use boundary particles but rather uses a distance function to represent the boundary. Therefore, the shape of a boundary can be represented precisely without increasing the resolution of the particles. Thus, the total particle number can be reduced and it lighten computational burden. Hence, the proposed method is useful for real-time applications.