The authors developed an axillary crutch walking trainer for an untrained patient. Up to now, it was confirmed that an untrained patient could acquire a skill of regular axillary crutch walking with the developed trainer. The trainer warns immediately the patient against the inadequate usage, that is, the axillary crutch was leaned more than the threshold angle while walking. In this paper, a method of calculating an acceleration of the body of a patient was proposed, so that a patient could be warned in case the possibility of falling is estimated. Because while a leg is in swing phase, the acceleration of the body of a patient affects the balance of the body after landing, it is important to calculate the acceleration. Although the acceleration of the body can be measured by an acceleration sensor attached to the body, the attachment may disturb a smooth walking. Therefore, a method of estimating the acceleration of the patient's body from an axillary crutch walking trainer's motion without an acceleration sensor was proposed. Through experiments, usefulness of the proposed method was confirmed. Additionally, possibility of estimate of ZMP was confirmed.