This paper presents a rigid piezoresistive sensor to measure cellular traction forces. The traction forces on a rigid substrate such as bone (10∼30 [GPa]) are still unknown. Our approach of the traction force measurement is not by an observation [1-4] but by a resistance change of the piezo-resistor due to the deformation of the rigid sensor (spring constant: 900 nN/μm). Because of its high sensitivity with sub-μN resolution, we were able to measure the traction forces on the rigid substrate. The stiffness of the proposed sensor was 10 times larger than that of previous works. We demonstrated that the traction forces generated by fibroblast were 0.7 μN when two cells adhered to the sensor.