A decade has passed since the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident on March 11, 2011. However, radioactive particles have recently been detected in the indoor air of some residences near the FDNPP. Following the recommendations of previous research, we determined the presence of radiocesium-bearing microparticles (CsMPs) and measured the radioactivity of radiocesium that adhered on non-woven face masks worn by six persons during the indoor cleaning of 59 residences in Namie, Futaba, Okuma, and Tomioka towns in Fukushima Prefecture. Of the 284 masks worn in this study, significant137Cs radioactivity was detected in 268, and 44 new CsMPs were discovered in 28. The results of this study also suggest the presence of highly concentrated soluble radiocesium particles or soluble radioactive cesium aerosols adhered to house dust. This implies that the CsMPs constituted a large proportion of radioactivity in the indoor air contamination for particles in the 1.0–2.5 µm size range due to the radioactive radiocesium particles. It is desirable to wear masks during cleaning to prevent inhalation of CsMPs.