The use of a scanning probe microscope (SPM), such as a conductive atomic force microscope (C-AFM) has been widely reported as a method of failure analysis in nanometer scale science and technology [1-6]. A beam bounce technique is usually used to enable the probe head to measure extremely small movements of the cantilever as it is moved across the surface of the sample. However, the laser beam used for a beam bounce also gives rise to the photoelectric effect while we are measuring the electrical characteristics of a device, such as a pn junction. In this paper, the photocurrent for a device caused by photon illumination was quantitatively evaluated. In addition, this paper also presents an example of an application of the C-AFM as a tool for the failure analysis of trap defects by taking advantage of the photoelectric effect.