Biomechanical analysis was performed to evaluate the effectiveness of mastering ukemi in preventing severe head injury in judo. One judo expert (tori) threw another judo expert (uke) with a skilled break-fall (ukemi) four times. We obtained kinematic data of uke with a digital video camera. Both translational and rotational accelerations were measured with a six-degree-of-freedom sensor affixed to uke’s forehead. When Osoto-gari was performed, uke fell backward and his arm made contact with the tatami; the translational and rotational accelerations rose to peak values. The peak resultant translational and rotational accelerations were respectively 10.3 ± 1.6 G and 679.4 ± 173.6 rad/s² (mean ± standard deviation). Furthermore, when comparing the values obtained for the judo experts with those obtained using an anthropomorphic test device (ATD: the POLAR dummy) that did not perform ukemi, both the peak resultant translational (P = 0.021) and rotational (P = 0.021) accelerations of uke were significantly lower than those for the ATD, whose head struck the tatami. Additionally, there was no significant difference among the three axis directions for either translational (a_x: 7.4 ± 0.2, a_y: 8.5 ± 2.1, a_z: 7.2 ± 0.8 G) or rotational (α_x: 576.7 ± 132.7, α_y: 401.0 ± 101.6, α_z: 487.8 ± 66.6 rad/s²) acceleration. We confirmed that performing correct ukemi prevented the elevation of head acceleration by avoiding head contact with the tatami when a judoka is thrown by Osoto-gari. Judoka should therefore undertake intensive practice after they have acquired ukemi skills.