Since the production of scallop aquaculture in Japan has increased largely, the automatic drilling machines has been used by the almost aquaculture bodies. Because of shell shape varying on size by growth and non automatic control mechanism built in drilling machines, severe problem has occurred that the machines always can not succeed to make a hole in the ear portion on a shell as expected. In this paper, the three respectable drilling areas were defined near by the byssal notch and by the ligament on the anterior ear of a shell. After confirmed of these parts obeying allometric growth, the successful rate (P) to drilling was estimated by some geometrical non-dimensioned parameters based on the vertical shell height (VH). The results of simulation were revealed as follows. 1) Near by the byssal notch : P was over 70% in case of VH over 60mm with 1.4-2.0 mm diameter of a drill chip (d), and P became up to 92% with VH=70mm and d = 1.6mm as normal use. 2) On the anterior ear : P reached to over 99. 6% with VH=40-55mm and d=1. 5- 2.0mm, also P showed to 99. 9% with VH=50mm and d=1.6mm for nomal use. 3) By the ligament : P was ranged from 30 to 77%, especially, P=70% under the nomal operation as same as 2). To locate the center of a drill chip according to the vertical shell height would rise up the successful rate to drilling through the ear part on a shell more higher than now.
