An RF-chopper possesses merits in both its high deflecting field and compactness. For this reason, it is suitable for chopping a high-current beam in a medium-energy beam-transport line (MEBT) of an ion linear accelerator. The JHF linac, as a high-current H - accelerator with an average current of up to 0.2 mA or higher in the second phase, took these advantages of the RF-chopper in its design. Two RF-deflecting cavities as the chopper will be used in the MEBT just downward of the 3-MeV 324-MHz RFQ. A 324-MHz RF-chopper cavity has been designed with optimization for a fast rise/fall time, which is an essential requirement for the chopper in a high-current accelerator in order to avoid radioactivity induced by the lost particles due to insufficient chopping during the transient time. The rise time can be less than 8 ns by means of a pair of ports with large coupling loops. In this paper, the details concerning the R&D of the RF-chopper are presented. Some RF properties calculated with MAFIA and HFSS codes have been compared with the test results of a cold-model cavity, showing good agreement. To cut down on the required RF-power, two RF-deflecting cavities are connected with a coaxial line in the design. The simulation and measurement of the coupled system exhibit a quick rise time that is almost the same as that of an independent cavity. The designed RF-chopper will be used in the JHF linac, which is under construction.