The invention provides a pressure sensor chip. The chip can improve the fracture toughness of the joint between the stopper member and the sensor diaphragm, and will not cause damage due to slight deformation of the sensor diaphragm caused by low pressure. An annular wall (1-5) is provided in a peripheral portion (1-2b) of the stopper member (1-2). The inner wall surface (1-5a) of the annular wall(1-5) faces to a space (1-4) which is formed by a concave portion (1-2b) surrounded by a curved surface of the sensor diaphragm (1-1) and the stopper (1-2). As a result, a step (a very small step) (h) is formed at the joint between the stopper member (1-2) and the sensor diaphragm (1-1) across the entire circumference, and the stress is dispersed by the step (h).