With the spread of smartphones, the number of accidents related to walking while looking on a smartphone is increasing. In this research, we propose a system to notify the danger of collision with a smartphone in order to prevent a collision accident with a pedestrian. While the user is using his smartphone, the proposed application runs in background and monitors surroundings via the smartphone camera. The warning notification is delivered by displaying warning images on the screen or by the notification sound. Experiments were performed to verify the effects of the proposed system. As a result of experiments, the collision rate when proposed system was used was lower than collision rate without the system and safety of system was confirmed.