We developed an automatic processing system for contact loss signal which outputs contact loss ratio and contact loss time, etc. This system is composed of a contact loss detection device, which outputs an contact loss signal from the input of pantograph current signal, and a personal computer in which A/D and D/A converters are installed. From field test results, we confirmed that contact losses were detected on a real time basis and that the contact loss ratio and the contact loss time, etc. were outputted to the display and recorder with high response.