Passive diffusion sampling has been widely used to monitor organic and inorganic pollutants in the environment. We herein developed a new and simple passive diffusion sampling method to monitor ¹⁴C in the indoor working environment of radioisotope facilities. The badge-type Shikata sampler is an open-end type with no diffusion cap that fits a collection filter with a diameter of 25 mm. This sampler could efficiently collect indoor ¹⁴C with good reliability and a collection efficiency that was 10-fold higher than that of the conventional active pumped method. The Shikata sampler is small, simple, and easy to handle. It may be subsidiary used to monitor indoor ¹⁴C concentrations in radioisotope working rooms, in which radioactivity must be measured once a month as recommended by the Industrial Safety and Health Law.