The purpose of this study was to confirm quantitatively the effect of foot orthotics on the reduction of ground reaction forces during running. Twenty-seven volunteers participated in this study. Ground reaction force (GRF) data were obtained from the subjects while running in two insole conditions (control and orthotic ones). The foot orthotics used in this study had mechanical functions to support medial longitudinal arch and rearfoot. Peak sideward GRF around heel-contact significantly reduced at 32% for the orthotic condition compared to the control one (P < 0.05). Cluster analysis based on the time series of sideward GRF data enabled to categorize the subjects into four representative groups. Peak sideward force significantly reduced at 54% for the orthotic condition (P < 0.01) for the group in which the subjects showed high heel-contact impact (more than 2.5 N/kg). These results suggested that well-designed foot orthotics could effectively reduce sideward impact force around heel-contact during running.