Degradations in attenuation images were evaluated by experiment using agar gel phantom and by computer simulation. As a result, it was determined that large errors on reconstructed attenuation constants are due to refraction of ultrasound. It was also confirmed that the influence of refraction can be corrected by measuring the maximum value of intensity distribution. Although this correction method is not completely accurate as reflection is not considered, it was proved to be practical from the results of the experiments and computer simulation.