This paper investigates a managerial problem related to human resource planning for an IT service company which mainly carries out time-base projects. As the demand for new workers is subject to wide fluctuations, the firm should hire workers in advance and train them. However, the firm should urgently recruit skilled workers from outside the firm due to the shortages in the workforce. Hence, it is important for the firm to design an optimal human resource planning program so as to fulfill the needs of new IT service projects and minimize operation costs, though this involves a trade-off between holding excess workers to prepare for upcoming demand and recruiting experienced workers. This paper presents a quantitative model that describes the stochastic behavior of the supply and demands of the workforce. Numerical results pertaining to the optimal solution are given via a simulation.