This paper describes a safe driving support system of an electric wheelchair expected to be used by elderly people. However, accidents involving electric wheelchairs tend to occur when they are incorrectly operated. Improving safety and utility is a very important problem. To solve the problem, we propose a new safe driving support system, which controls a wheelchair semi autonomously based on the degree of danger. The degree of danger is calculated by fuzzy reasoning from both the manually controlled input from a joystick and the time to collision. Experimental results using our developed electric wheelchair demonstrate the high control performance of the proposed system.