The purpose of this study is to develop and apply an oil leak detector using a capacitive sensor to detect oil leak in hydraulic equipment. The developed oil leak detector consists of a sensor and a sensing circuit. The sensor is designed using the difference in the permittivity of air and oil to change the capacitance, and the sensing circuit is composed of a charge amplifier and rectifier circuit. The sensing device is made of a PCB module to output the DC analog signal. In this study, this oil detector was installed in a cyclic pressure tester for evaluating valve life and was applied to detect the leakage of the test valve. It can also be applied to detecting the oil leakage of various hydraulic types of equipment and reduce maintenance costs by preventing large leakage of hydraulic oil.