The cellular automaton method, which can generate the Navier-Stokes equation, is utilized for simulating fluid motion numerically. These partial differential equations are solved with boundary conditions specified by an actual situation. Various flow patterns are highly dependent on boundary conditions. The practical inflow and outflow parts in the open system are idealized by the Neumann or Dirichlet conditions. For automaton situations, the periodic boundary condition is often introduced due to its advantage of keeping exact mass conservation. The present paper proposes a microscopic expression for the inflow and outflow boundaries as Dirichlet and Neumann condition on the boundary. This microscopic expression was applied to the Poiseuille and backstep flows to assure its validity and feasibility of a microscopic expression.