A vesicle that exhibits oscillatory structural changes under a pH gradient is reported. This vesicle is composed of oleic acid and sodium oleate and appears to exhibit a stomatocyte-type or a seemingly double spherical shape initially. The inner water is located at one side of the double spherical structure (i.e., the swollen part). The other side of the vesicle is nearly free of water. When the diffusion of hydroxide anions causes a pH gradient, the double spherical structure rotates such that its swollen part faces the higher-pH side. After this event, a small hole in the center of the swollen part begins to open. The vesicle changes to a torus-like structure: the swollen part is formed at the periphery, and a disk-like part is formed in the central part of the torus. The shape change proceeds via the torus-like shape, and the vesicle closes again to form a double spherical shape with a curvature opposite to the initial curvature. At this stage, the swollen part faces the lower-pH side. After this event, the vesicle rotates back to its initial state, where the swollen part faces the higher-pH side. This rhythmic transformation repeats numerous times. The size of the vesicle remains nearly constant during these numerous transformations. Thus, this vesicle continuously generates mechanical work from a pH gradient. In this study, experimental observations under an optical microscope and a model for the transformation are reported.