The purpose of this study is to analyze the effects on the insertion path of an ultrafine needle injected into a tissue under the pressing force of an ultrasonic probe. We developed a hypothesis that the insertion path is affected by the stress field and the insertion angle that occurs with the deformation of the tissue. To examine the effect of each factor, we compared the deflection of the needle caused by both the stress field and the insertion angle with that caused only by the insertion angle. The results revealed that the insertion angle is a major cause of variation in insertion path. Furthermore, it is possible to reduce the variation in the insertion path using a guide to restrict the insertion angle with the deformation of the tissue.