This paper presents a new method and device for protein presence and size detection based on the coordinate shift of the rigidity changing points due to proteins. Compared to the conventional resonant method, the present nanomechanical method shows higher precision. The present method also offers simple and inexpensive protein detection by removing labeling process and optical components. We design and fabricate the nanomechanical protein detector using an electrothermal actuator and a nano-gap. In experimental study, we measure two rigidity changing points and their coordinate shift between two cases (without/with target protein). The fabricated device detects the protein presence and size of 14.0±7.4 nm from this coordinate shift of rigidity changing points. We experimentally verify the protein sensing capability of the nanomechanical protein detector, applicable to high-precision protein detection.