High-intensity focused ultrasound (HIFU) surgery is one of the non-invasive therapeutic methods. The high temperature generated by this method coagulates the target non-superficial tissue. In this study, we evaluate the effect of thermal coagulation by HIFU on acoustic impedance. Chicken breast muscle was used as the specimen. Acoustic impedance was measured in cross sections of the HIFU-exposed specimen using an ultrasonic microscope. The acoustic impedance of the coagulated region was significantly lower than that of the non-coagulated region.