This report deals with the scanning acoustic microscope for imaging cross sectional acoustic impedance of biological soft tissues. A focused acoustic beam with a wide frequency range up to about 100 MHz was transmitted to the tissue object in contact with the "rear surface" of plastic substrate. The reflected signals from the target and reference are interpreted into local acoustic impedance. Two-dimensional profile is obtained by scanning the transducer. As the incidence is not vertical, not only longitudinal wave but also transversal wave is generated in the substrate. The error in estimated acoustic impedance assuming vertical incidence was discussed. The error is not negligible if the angle of focusing is large, or the acoustic impedance of the reference material is far different from the target. However it can be compensated, if the beam pattern and acoustic parameters of coupling medium and substrate were known. The improvement of precision brought by the compensation was ensured by using a droplet of saline solution of which acoustic impedance was known. Finally, a cerebellum tissue of rat was observed with a good precision.