Passive acoustic mapping (PAM) is a promising method for imaging cavitational acoustic emission induced by high-intensity focused ultrasound. In this paper, we present a portable imaging system capable of monitoring the intensity map of cavitation in real-time. Based on PAM in the frequency domain for implementing passive beamforming in a low-cost field-programmable gate array (FPGA), it is designed to use as few resources as possible in the calculation process. The probe system, consists of analog and digital front-ends, performs PAM and B-mode beamforming. Images made by beamformed data are shown in the graphical user interface (GUI). Parameters and modes can also be changed in the GUI. As a result, the system can provide real-time PAM images using one selected frequency. The accuracy and stability of the system have been verified through experiments with a high-intensity focused ultrasound (HIFU) transducer and a linear array transducer.