To suppress the lowering of the yield after a pressurized heat-treatment and reduce the deterioration of the characteristic palatability and taste of the fish and shellfish by pretreating the fish, and the like, to be subjected to the pressurized heat-treatment such as retort sterilization.

The pretreatment is carried out by immersing the fish or shellfish to be subjected to the pressurized heat-treatment in a salt water produced by concentrating seawater by ion exchange membrane treatment, or the like, (e.g. salt water having a salt concentration of 2-20% and pH 5.0-8.0) and boiling the fish or shellfish in hot water or in the salt water before the pressurized heat-treatment.