To provide a radar detector which is built in a helmet, can especially inform the presence of a speed-measuring device to a rider, and can be mounted on the rider only by putting the helmet on the head of the rider.

This radar detector is made by attaching a solar battery 5 to the upper portion of the helmet 1, attaching a battery to the inside of the helmet 1, and attaching a radar detector main body 7 to the side of the helmet 1. The radar detector detects electric waves emitted from a speed measurer to know the position of the speed measurer, and informs the presence of the speed measurer to the rider with sounds or displays. The parts are attached to the helmet 1, and the radar detector can be mounted by putting the helmet on the head of the driver. The extremely easily handleable radar device built in the helmet is thus provided.