To achieve the shortening of a sieve working time at a time of the sampling of benthos, the enhancement of the accuracy of sieve work, the reduction of the number of workers and the reduction of worker's fatigue and to prevent the pollution of the surface of the sea caused by the discharge of muddy water.

A deposit container 1, a mud suction device 2, an air compressor 6, an air piping hose 7, a mud/seawater suction pipe 9, a benthos automatic sieve 9 and a stainless steel mesh 10 are provided and a mixture of mud 3 and seawater 4 is sucked by the compressor and seawater is returned to the deposit container 1 from a seawater discharge port 14 and the benthos 11 remaining on the stainless steel mesh 10 is sampled and residual sludge is taken out of a mud taking-out port 15.