The three-dimensional movement of a tooth, any tooth, while naturally eating, was measured by an apparatus developed in our laboratory. The movement of the first molar on the usually used side is reported here. The mastication movements are divided into two parts, namely the rhythmical chewing period during which there are mastication pressure loads on the molar and the irregularly pulsated moving period in preparation for the swallowing followed by the rhythmical one during which there is no more pressure on it. The amplitudes of the up and down movements of the molar are 10–22mm for the rhythmical chewing period which are bigger than those for the irregularly moving period. The rhythmical chewing periods are distributed around 10–40 s depending on 14 kinds of food texture, and also on the size of the food, in a fairly similar manner for the participants. The period is longer for tough foods and bigger samples. The irregularly pulsated moving periods are widely distributed, 2–45s, and depend both on the food texture and on the participant. The irregular moving periods are longer for foods which scatter in the mouth or adhere to the teeth when chewing. The ingestion difficulty of the food was divided into two groups. One is where it is difficult to break down the food with the teeth and the other is where it is difficult to prepare for swallowing.