A determination assist system for use in ultrasonic testing, adapted to generate a determination image used to determine whether or not porosity is present in a composite material component, based on data of plural kinds of test indices obtained in an ultrasonic testing device, the determination assist system comprising: a first image generating section adapted to generate a first planar image which is a gradational image, based on data of signal intensity of a reflected sound wave and identify a portion where the porosity is present, with reference to a determination image generated based on the first planar image; a signal intensity difference deriving section adapted to derive a difference between the signal intensity of the reflected sound wave in a portion of a bottom surface of the composite material component and the signal intensity of the reflected sound wave in the portion where the porosity is present; and a thickness obtaining section configured to obtain data of a thickness of the composite material component according to a propagation time of an ultrasonic sound wave; characterized by a deriving section adapted to derive a degree of the porosity in the portion where the porosity is present, based on the derived difference of the signal intensity and the obtained data of the thickness, with reference to a predetermined porosity rate conversion graph defining a correspondence among the thickness, the difference of the signal intensity in the portion where the porosity is present, and the degree of the porosity in the portion where the porosity is present; wherein the thickness obtaining section includes an interpolation section which is configured to interpolate data of a thickness of a region of the composite material component for which a reflected sound wave from the portion of the bottom surface of the composite material component is not displayed clearly in the first planar image, based on data of a thickness of a surrounding region of the said region in the composite material component; and wherein the deriving section is configured to derive the degree of the porosity based on the interpolated data of the thickness of said region, and the difference of the signal intensity. a first image generating section adapted to generate a first planar image which is a gradational image, based on data of signal intensity of a reflected sound wave and identify a portion where the porosity is present, with reference to a determination image generated based on the first planar image; a signal intensity difference deriving section adapted to derive a difference between the signal intensity of the reflected sound wave in a portion of a bottom surface of the composite material component and the signal intensity of the reflected sound wave in the portion where the porosity is present; and a thickness obtaining section configured to obtain data of a thickness of the composite material component according to a propagation time of an ultrasonic sound wave; characterized by a deriving section adapted to derive a degree of the porosity in the portion where the porosity is present, based on the derived difference of the signal intensity and the obtained data of the thickness, with reference to a predetermined porosity rate conversion graph defining a correspondence among the thickness, the difference of the signal intensity in the portion where the porosity is present, and the degree of the porosity in the portion where the porosity is present; wherein the thickness obtaining section includes an interpolation section which is configured to interpolate data of a thickness of a region of the composite material component for which a reflected sound wave from the portion of the bottom surface of the composite material component is not displayed clearly in the first planar image, based on data of a thickness of a surrounding region of the said region in the composite material component; and wherein the deriving section is configured to derive the degree of the porosity based on the interpolated data of the thickness of said region, and the difference of the signal intensity.