Cycle-resolved measurements of the fuel concentration near a spark plug in a practical spark-ignition (SI) engine have been developed. An in situ laser infrared (IR) absorption method was applied using a spark plug sensor and a 3.392-μm He-Ne laser as the light source. The newly developed IR spark plug sensor had a higher signal-to-noise ratio than its previous version due to the optimization of its sapphire lens and two optical fibers. Firstly, a database of the molar absorption coefficients of regular and premium gasoline at different pressures and temperatures was established in advance using constant-volume vessel. The molar absorption coefficient of gasoline decreased with increasing pressure above atmospheric pressure. Secondary, the measurement accuracy was investigated in a constant-volume vessel and a compression-expansion machine. Thirdly, fuel concentration measurements in a spark-ignition (SI) engine with ethanol blended gasoline were carried out using an optical sensor installed in the spark plug with laser infrared absorption technique. Mixture formation processes of ethanol blended gasoline were investigated using spark-plug sensor installed in a spark plug in a port-injected SI engine with changing the fuel injection timing in an intake port.