This paper proposes optimal design parameters that maximize the heartbeat signal magnitude of the Photo plethysmography (PPG) sensor case through the robust design. In this study, we designed the case to obtain the desired output in various user conditions by using this Taguchi method. For robust design, we defined sensitivity as the difference between the maximum and minimum points of the heartbeat signal for one cycle. And, the compressive of 1N, 3N and 4N between the sensor case and the contract point is designated as the user conditions. In addition, design parameters consist of factors which affect absorption and reflection of light. The first factors are x, y-axis of Light Receiving Diode(L.R.D_x, y). Second factors are x, y-axis of Light Emitting Diode(L.E.D_x, y). Third factors are height (H) and color. The experimental results show that the design parameters are optimized when the design parameters of the PPG sensor case are H: 1.5mm, L.R.D_x: 5mm, L.R.D_y: 10mm, L.E.D_x: 8.5mm, L.E.D_y: 7mm, Color: Black. As the result of the final test using optimized design parameters, the signal differences of maximum and minimum heart rate in each user condition were 4.85 in condition 1, 4.90 in condition 2, and 4.89 in condition 3.