We developed a physiological method to open oyster shells persistently for use in an automated oyster shucking machine. When oysters (shell height, 10-13cm; shell length, 5-7cm; molluscous body weight, 9-22g) were submerged in 0.369m MgCl₂ solution, their adductor muscle began to relax persistently after 17±5.9min (mean+standard deviation) at a water temperature of 5°C and after 18±11.5min at 15°C. The muscle did not contract when the molluscous body was stimulatedwith a needle. After the oysters were transferred into artificial sea water, the muscle began to contract and the oysters resumed shell movement in 78±44.2min at 5°C and 24±5.4min at 15°C. When 120 individuals were submerged in MgCl₂ solutions at 17-18°C, cumulative persistent shell opening rates in the solutions of 0.369m and 0.738m were 14.2% and 63.3% for 30min submersion, 35.0% and 72.5% for 60min submersion, 64.2% and 83.3% for 120min submersion, and 76.7% and 85.8% for 180 minsubmersion, respectively. This method for opening oyster shells persistently with the use of Mg²⁺ will be applicable to other bivalves for opening shells by adjusting the Mg²⁺ concentration and submersion time for each species.