Biomechanics of the cell indicates the inner structure and viability of the cell. Mechanical properties are represented by acoustic properties such as speed of sound (SOS) or acoustic impedance. In the present study, cellular resolution scanning acoustic microscope combined with optical microscope (OptSAM) is developed to observe the change of mechanical properties in cell differentiation. Main part of the OptSAM was consisted of 350 MHz ultrasound transducer mechanically scanned by a piezo-actuator. Thickness, SOS, acoustic impedance, density and elastic bulk modulus of the cell were deduced by the ultrasound responses in both time domain and frequency domain. C2C12 cell changing its form from myoblast to myotube was observed by OptSAM. The value of bulk modulus slightly increased in response to differentiation process. OptSAM non-invasively provides important information on biomechanics of cells without contact or staining.