Reducing friction in the crankshaft main bearings is an effective means of improving the fuel efficiency of reciprocating internal combustion engines. To realize these improvements, it is necessary to understand the lubricating conditions, in particular the oil film pressure distributions between crankshaft and bearings. In this study, we developed a thin-film pressure sensor and applied it to the measurement of engine main bearing oil film pressure in a 4-cylinder, 2.5 L gasoline engine. This thin-film sensor is applied directly to the bearing surface by sputtering, allowing for measurement of oil film pressure without changing the shape and rigidity of the bearing. Moreover, the sensor material and shape were optimized to minimize influence from strain and temperature on the oil film pressure measurement. Measurements were performed at the No. 2 and 5 main bearings. Results confirmed that main bearing oil film pressure is primarily influenced by the combustion pressure of adjacent cylinders, but the range of influence varies between bearings. For comparison with measured results, we calculated bearing load and oil film pressure distribution for each bearing. The analysis showed that oil film pressure is affected by housing stiffness and crankshaft bending rigidity, resulting in a different load distribution at each bearing. Analysis also showed that deformation at the sliding surface induces changes in the bearing load distribution in the direction of rotation.