The method of calculating the shear strength of pile cap of caisson foundation is established by the researches in recent years. As they are based on the experiment at the shear span ratio of 0.5 or over, however, the shear strength of RC members at he shear span ratio less than 0.5 is not known. Then, a method of calculating the shear strength was studied based on the result of an experiment that used a model of pile cap for caisson foundation. As a result, it was clarified that the shear strength of pile cap for caisson can be calculated at a very small shear span ratio, if the present calculation method is corrected.