Mounting angle detection device

A mounting angle processor (4) for determining a mounting angle of a vehicle-mounted device (101) based on an acceleration sensor (1), a gyroscopic sensor (2) and a GPS receiver (3) mounted in the vehicle-mounted device, and a comparator (6) for comparing the determined mounting angle and a permissible range corresponding thereto are provided. If the determined mounting angle is out of the permissible range, it is determined that themounting angle is abnormal. Thus, a mounting angle detection device is provided for preventing the vehicle-mounted device from being mounted at an improper angle, or for maintaining the accuracy of the vehicle-mounted device such as a navigation device (101) even in the case where the mounting angle of the vehicle-mounted device has been changed.