[en] The movable radioactivity measurement device was developed to evaluate a low-level radioactivity contamination distribution at the final stage of dismantling activities in decommissioning nuclear facilities. The detection system was designed to quantify only β-rays by separating from γ and β-rays emitted from ⁶⁰Co and others. The detector was structured in the form of collecting twelve unit plastic scintillation counters, to identify relatively small partial contaminations. It was found from the performance tests that the detection limit for Co-60 was about 0.1 Bq/cm² with 60 seconds of the measurement time, and that an external radiation such as from embedded pipes can be discriminated. The measurement device has capability to measure contamination distribution with 4-5m²/h in efficiency, which was three times faster than hands-on work using conventional detectors. (author)