To improve the performance and efficiency of rock drills, it is essential to understand the penetration behavior of a bit into rock. However, studies on the impact penetration with a button bit, which is commonly used for percussive drilling of hard rock recently, are insufficient. In previous studies, large fluctuations were observed in a force-penetration curve calculated from rod stresses measured during percussive drilling, and therefore precious force-penetration curves could not be obtained. In this study, a calculation method was examined to preciously obtain a force-penetration curve from impact penetration tests. The results of numerical simulation showed that the fluctuations can be caused by the mismatch between the product and calculation model, and that the precious forcepenetration curve may be obtained with subtracting the bit force in a free end test from that in an impact penetration test. The suggested method was applied to the measured results of impact penetration tests with granite conducted by Fukui et al. (2010), and improved force-penetration curves were obtained. The method proposed in this study can be adopted to in-situ rock drilling, and contribute to the development of the bit suited to each rock type.