A method to estimate mass sedimentation rate is proposed in the following procedure; 1. drafting of isopach map of sediment based on the depth of specific reflector in acoustic (UNIBOOM) record, 2. making vertical prone of standard water content in sediments at selected point(s) in the area, 3. conversion of the data of water content into those of dry weight and cumulative dry weight of sediments, 4. age determination of the critical reflector, (in general, some of reflectors are age-known tephras.) 5. making a standard curve of " thickness versus sedimentation rate" through dividing cumulative dry weight in each depth by sedimentary age of the reflector, 6. conversion of thickness of sediments into sedimentation rate by using that curve. In Lake Biwa, depths of reflectors beneath the bottom coincide with those of the top of corresponding sediment layers which contain more than 1% of sand. Consequently, thickness of sediment can be caluculated easily based on the acoustic record. Vertical profile of water content is obtainde using drilled samples in the area. It is converted into that of dry weight and cumulative dry weight profiles, assuming that specific gravity of sediment grains to be 2.65 and that all pores in sediments are filled with water. In Lake Biwa, the top of Kikai-Akahoya tephra (K-Ah; aged ca. 6300 Y. B. P.) makes a good reflector. Mean sedimetation rate in the whole area of Lake Biwa, obtained by this method, is 38.2 mg/cm_2/year. Comparison between the results and the rates by 210Pb, 137Cs and specified tephra methods shows good correlation, with a few exceptions. The result of isotope method show a little smaller values than those of acoustic method. There are several factors to make an error in this methd. They are; 1. difference between sediment thickness obtaind through acoustic record and the real thickness, caused by vertical change of acoustic velocity (This is, however, negligible in Lake Biwa.), 2. areal difference of water content, 3. difference in degrees of compaction with time and depth, 4. age determination error (As for 14C dating, they are within a few percent.), 5. sporadic sedimentation, such as turbidite, 6. change of sedimentation rate in recent years caused by the anthropogenic deformation of surrounding land area. As there are many active and non active volcanoes in Japan, many volcanic ash seams can be found in the lake and shallow sea sediments. Consequently, this method will be applied in various lakes and shallow sea areas with muddy sediments.