The adhesive protein of the mussel widely living in the ocean is expected as an ingredient of the adhesive. Adhesive strength of the protein to substratum has not been clarified because a method of measuring the strength does not exist. In this research, the plaque consisted of adhesive protein was peeled using the spatula-shaped jig inclined at the angle of 45 degrees to substratum. Adhesive strength was determined from the peeling load obtained by the jig with a strain gauge. To investigate the effect of the thickness of the plaque on the peeling load, based on linear fracture mechanics, peeling simulations of plaque from substratum were conducted using the finite element method. The simulation suggested that adhesive strength of the plaque to substratum is quantitatively evaluated by averaging the peeling load fluctuations.