Waste gases containing radio-isotopes (RI) produced in the synthesis of PET (Positron Emission Tomography) tracers are stored in RI gas storage devices to allow radioactive decay to an acceptable level. Relatively large sizes of the RI gas storage devices, however, prevented us from placing them in a hot-cell (small shielding chamber). In this study, we developed a compact RI gas storage device for placing in a hot-cell, in order to provide a facile means to achieve a safe environment for the synthesis of PET-tracers. We designed our RI gas storage device limiting the number of the usage times a day and optimized the capacity of each component. Consequently, we succeed to downsize the device to W555×D630×H850 mm, which enable us place the device in our hot-cell. A preliminary experiment of synthesizing a 18F-labeled compound proved the capacity of our device to store the RI gases.