An investigation on the dust in depositories, exhibition rooms, and showcases of the Nara National Museum was carried out by using a particle counter of Climate Controlled Environmental Monitor Model C1-250. The air sent into the depositories had sufficiently been cleaned by air conditioners. The concentration of dust in them was 1/10 to about 1/100 of that of the air outside. When a party of 400 visitors stayed in the exhibition room, the concentration of dust in it rose 10 times, but dropped to one third in 15 to 20 minutes after the party left.