Since the 1990s, research analyzing the impact of human resource management (HRM) systems on organizational performance has been actively conducted. Prior research findings have demonstrated that high-performance HRM systems have a positive effect on organizational performance that is significant not only statistically but also economically. However, it is necessary to examine whether the approach adopted by prior studies to calculate the high-performance HRM system index—that is, the additive method that simply sums up the implementation intensity of individual practices under the assumption that a linear relationship exists between the individual practices constituting the high-performance HRM system and organizational performance—is appropriate. This is because some practices known to be components of high-performance HRM systems have been found to have non-linear relationships with organizational performance. This study applied the profile deviation method, an alternative approach for calculating the high-performance HRM system index, and confirmed the existence of a causal relationship between high-performance HRM systems and organizational performance. Furthermore, the study confirmed that when some high-performance HR practices have non-linear relationships with organizational performance, the profile deviation method captures the marginal effects on organizational performance better than the additive method when calculating the high-performance HRM system index. The significance and limitations of this study are presented in the conclusion and discussion section.