Surface pressures and shear forces were measured in order to clarify the mechanism leading to the development of a pressure ulcer at five sites on the body during the operation of a bed (bed) using a device for simultaneously measuring pressure and shear force. Changes of shear force and pressure when three body types adopted different supine positions, with or without raising/bending the knees (raising the knees), were investigated and analyzed. The results are as follows: a slender body type tends to have the highest shear force at the coccygeal bone site and also has a higher surface pressure at the coccygeal and at the lateral sacral bone sites than an obese body type. On the other hand, an obese body type has a higher surface pressure at the other sites than the slender type. Shear forces at the sacrum and coccygeal bone sites can be reduced during a bed operation by raising the knees. Furthermore, shear forces can be reduced during a bed operation in a supine position by matching the body's bending points with those of the bed or by shifting the subjects 10 cm toward the head of the bed. These new findings are clinically useful in the treatment and the prevention of the onset of pressure ulcers.