This paper describes a 2D detection method of single metal nanoparticles based on photothermal effect. Recently, it has been recognized that in order to utilize metal nanoparticles for optical labels, a real-time 2D detection method of individual metal nanoparticles is desirable. Metal nanoparticles can be detected by the photothermal effect, which is converting light energy to thermal energy, and localized surface plasmon resonance (LSPR). The photothermal effect induced by LSPR makes it possible to magnify an image of individual nanoparticles apparently. Its image can be detected using a single element interferometer for robust detection and performing phase analysis using the Fourier transform method for real-time detection. This paper reports that the phase of interference fringe shifted by the photothermal effect and LSPR was obtained by use of single element interferometer and the Fourier transform method. Furthermore, measuring phase shift induced by photothermal effect and LSPR, locations of single gold nanoparticles were detected by time-resolved phase measurement.