A survey of pneumoconiosis caused by "send" dust was performed on the rush-mat workers in Seno-cho and Hayashima-cho of Binan district, Okayama Prefecture. The results obtained were as follows: 1) Out of 199 rush-mat workers in Binan district (of whom, 163 subjects with working career of over 10 years in Seno-cho and 36 subjects with career of over 15 years in Hayashima-cho), 2 cases of P(3), 7 cases of P(1) and 6 cases of P(2) were found. 2) Pneumoconiosis due to "sendo" is clay pneumoconiosis and atypical silicosis caused by inhalation of the dust with free silica in low percentage (about 20%). And this disease shows its gradual development. As is evidenced by these, most of the cases of pneumoconiosis due to "sendo" were found in the workers with working career of over 20 years. 3) It was examined that 48.7% of "sendo" dust contained particles of under 5μ in diameter. And a higher incidence of pneumoconiosis due to "sendo" of fancy-mat workers among rush-mat workers was considered to be caused by indoor labor at the ill-ventilated workshop filled with a great deal of "sendo" dust. 4) Accordingly, as preventive measures against "sendo" dust, there needs the improvement of the environment in the workshop to install a ventilation and so on.