[en] Economic effects at present have been investigated limiting to radioisotope-utilizing instruments for industrial application. These instruments have increased year by year since they were introduced into Japanese industry about a quarter century ago, and reached 6058 sets as of March, 1976, in all industrial fields. Most widely used instruments are thickness gauges, level gauges, densimeters, moisture gauges and sulphur analyzers in this order. Methods for analyzing the economic effects includes several systems, and here, the cost-benefit ratio (C/B) method, which is called IAEA system, was adopted. Investigation has been conducted only on thickness gauges for paper making, coating film thickness gauges, sulphur analyzers and moisture gauges. For the thickness gauges, which are used a lot in paper machines and now fully automated, C/B is 1/3.16. The coating film thickness gauges are being changed from β-gauges to fluorescent X-ray system, and its C/B is equal to 1/30 to 1/50, very economical. Though the sulphur analyzers for oil industry are used off-line, here the on-line analyzers were investigated. The result of C/B of 1/10 was obtained for the plant whose production is 2,000 kl/day. The neutron moisture gauges are employed a lot specifically in iron and steel industry. Its C/B is from 1/20 to 1/40 for an example used in a pig iron plant. As seen above, the radioisotope- utilizing instruments have very high economic effects, and can contribute to energy saving, labor saving and resource saving in process and quality controls. (Wakatsuki, Y