Behavioral synthesis, which automatically synthesizes an RTL circuit from a sequential program, is one of promising technologies to improve the design productivity. However, behavioral synthesis has not become popular yet in industry since the quality of generated circuits is not satisfactory, especially in the synthesis from the large programs with a number of functions. This paper proposes a method to optimize function calls in behavioral synthesis. We formulate the optimization problem using integer linear programming. Our experimental results show that our method reduces the circuit area by 44.6%, compared with a traditional method.