A passenger detecting apparatus determines the presence of a passenger or an object, such as a child seat, in a seat unit, estimates the weight of the passenger or determines the build of the passenger, thereby improving the usability of peripheral devices of a seat. The passenger detecting apparatus has a dielectric sensor (111) and a pressure-sensitive sensor (112) in a seat cushion (102) and detects the presence of a passenger or an object on a seat based on their outputs, or has a dielectric sensor and a pressure-sensitive sensor in the seat cushion and a dielectric sensor (113) in the back rest and an angle sensor (114) for detecting an angle defined by the seat cushion and the back rest and determines the build of a passenger based on their outputs.