By the past studies, tree root system has effects of preventing shallow landslides. In this study, pull‐out test for tree root was performed at constant speed, and changes with time of pull‐out resistance of tree roots was measured. Pull‐out test was carried out for the purpose of grasping effects of tree roots on shallow landslides appropriately. As a result, it became clear that the maximum value of tensile resistance increased and the time to show the maximum value was delayed, in proportion to the diameter of the root. Based on these results, the model considering changes with time of resistance to landslides was created. Comparing model values with measured values, it was confirmed that this model successfully reproduced measured results. In conclusion, compared with the conventional model, it has become possible to greatly correct effects of tree roots on preventing shallow landslides.