A method for installing an inertial sensor unit includes: attaching a substrate to a structure with a magnet; and attaching a case accommodating an inertial sensor, to the substrate. The case is provided with a first attachment part. The substrate is provided with a second attachment part. In the attaching the case, the first attachment part and the second attachment part are fitted together to attach the case to the substrate. The inertial sensor has a base part, a moving part coupled to the base part, a physical quantity detection element arranged at the moving part, and a mass part arranged at the moving part. The mass part is made of a non-magnetic material.