An apparatus for measuring the ratio of alcohol contained in mixed fuel which flows through a fuel pipe to the engine. The fuel pipe is made of a high frequency signal permeable material such as rubber, teflon or nylon, for example. A microwave chamber is provided outside the fuel pipe, enclosing a part of the fuel pipe. The microwave chamber comprises a pair of wave guides opposed to each other with the fuel pipe interposed between them. One of the wave guides is provided with an antenna section for transmitting microwaves received from a microwave generator. The other wave guide is provided with a receiving antenna section for receiving the microwaves permeating through the fuel pipe through which the mixed fuel is flowing. The microwaves passing through the fuel pipe are attenuated according to the amount of alcohol in the fuel. The microwaves received by the antenna section are detected by a detector and converted to DC voltage signals which correspond to the strengths of the microwaves received and thus the amount of alcohol in the fuel.