[en] Holdup MOnitor System (HMOS) was developed for the purpose of verifying the constant holdup during facility maintenance in Plutonium Conversion Development Facility (PCDF). The glove box assay system (GBAS; big slab) has been used by inspectors, measures the holdup periodically (i.e. IIV) using coincidence counting. The GBAS couldn't be used for inspection during maintenance period. Because many glove boxes (GB) set in process area had been occupied by large vinyl green-houses due to maintenance. We aimed that the holdup except for the maintenance GB should be constant during maintenance period, the HMOSs were set to 3 GBs. The system had been used from June '98 to July '99 for verification. The HMOS detector is located top/bottom of the GB, counts total neutron variation in the GB continuously. Detector efficiencies are 1.2%(top) and 0.12%(bottom), respectively. The measurement variation is observed up to 1.5%(3σ). The HMOS has high sensitivity 8 to 90g Pu (3σ; In case of 1kg Pu holdup, the sensitivity depends on position in the GB). The movement of equipment or nuclear material from/in the GB can be detected effectively. Though the HMOS observes measurement variation related to humidity in the GB, hygroscopic effect of denitration MOX powder, material/equipment movement and mainly ²⁴¹Pu nuclear decay, this system can verify that the holdup is constant qualitatively. As a result, in PCDF, safeguard related to the inventory verification during maintenance period (more than 1 year) were successfully performed using holdup monitor system. (author)