This paper describes the development of a personal warning system for preventing construction site accidents involving workers and heavy equipment. The required functions were established on the basis of a past accidents model. The system was designed to send a warning via wireless from a server computer to a worker and relevant parties according to estimation of distance between worker and equipment. RFID technology was used to estimate the distance. A warning message was defined as a means of sending information about danger to the worker and relevant parties. Parties receiving a warning were set up for each case, and the contents of the warning message were prepared accordingly. An experimental prototype device for sending a message from a server computer to a smart-phone was made under a wireless local area network, and it was clarified that to accept a warning message and to send a response.