This paper presents a design strategy for simple robotic spines for androids with a natural upper-body postural appearance. The strategy allows us to develop a robotic spine with a minimum number of required movable joints, granting androids the ability to adapt natural upper-body postures while performing various gestures. In this study, by restricting the motion of the spine to sagittal plane for simplicity, the minimum number of required movable joints was determined using measured human spinal motion data. The error between the measured data and the model with a limited number of movable joints was calculated and the required number of joints was determined depending on its value. The efficacy of the proposed design strategy was validated through impression evaluation with five subjects. We also developed a prototype with four movable joints using low-cost mass produced parts.