This paper is devoted to a detailed description of a device, an operation time monitor, which is used to investigate the behavior of the operation time in a digital computer. The principle employed to design the monitor is a very basic relation in fundamental electricity -the voltage across a capacitor is linearly proportional to the time during which a constant charging current is flowing through the capacitor. The constant current source which provides the required charging current is controlled by the pulses conveying the input information from a digital computer. Some data obtained by this monitor are included at the end of this paper to illustrate its application to a digital computer.