Knowledge of crustal stress field is essential for understanding tectonics and earthquake generation. One way to estimate the crustal stress field is based on focal mechanisms of earthquakes. This study investigated focal mechanisms of ~ 110 thousand microearthquakes in Japan Islands shallower than 20 km based on the first-motion polarities picked by a simple neural network model, which was trained using two data sets: moderate to large earthquakes all over Japan and microearthquakes in two regions in Japan. The threshold of the confidence score from the neural network model was so chosen as to maximize the overall quality of focal mechanism solutions. The P- and T-axes of the numerous focal mechanism solutions provide more detailed distributions of crustal stress field. For example, in Chugoku region, there exist slight differences in the trend of P-axes azimuths between the northern and southern areas are observed, corresponding to the geodetic observations in space.