This study aims to clarify the influence of the rate of high-temperature setting on the strength properties of sugi (Cryptomeria japonica) boxed-heart square lumber with pith. Past experimental data of strength properties of sugi boxed-heart square lumber with pith treated with different rates of hightemperature setting were analyzed by Bayesian statistical modeling. The data consist of bending strength and shear strength in relation to four different drying conditions, namely three different hightemperature settings for 24, 36, 48 h with dry-bulb temperature of 120 °C and wet-bulb temperature of 90 °C, combined with subsequent conventional kiln drying, as well as air-drying for 18 months. The developed model revealed that both the bending strength and shear strength decreased by hightemperature settings for more than 24 h and subsequent kiln drying. Based on the strength for airdrying, the rate of decrease in bending strength for the treatment times of 24, 36, 48 h was 7.6%, 8.3%, 21.3% on average, respectively. Meanwhile, the rate of decrease in shear strength for the treatment times of 24, 36, 48 h was 13.9%, 19.4%, 22.1% on average, respectively. These quantitative findings are useful in determining an appropriate high-temperature setting of treatment time for sugi boxed-heart square lumber with pith.