A first train-installed device according to the present invention includes: a memory; and a processor. The processor performs operations. The operations includes: performing wireless communication through an antenna installed at a front of a train; acquiring train information including information indicating a railroad of the train; transmitting a search signal to another train in a neighborhood of the train; when receiving a response signal including train information including information indicating a railroad on which the another train is on-rail, determining whether there is a possibility of a collision between the train and the another train based on the train information; and when there is a possibility of the collision, measuring a distance between the another train and the train by transmitting and receiving a measurement signal and a measurement response signal to and from the second train-installed device.