The authors have newly developed a data transmission device using conventional ATS-S type on-board equipment. This device enables inexpensive and convenient transmission of train information, such as train classification and train number, because data is transmitted from train to ground using the existing ATS-S on-board antenna, and this added function is easy to realize by attaching a transmission unit to the conventional on-board equipment. If this device is combined with the level crossing control system, the warning time will be able to control more constantly. In this paper, first, the construction and function of this data transmission device are explained. Following this, results of field tests are reported.