A signal processing apparatus comprising: a relative value calculation section configured to calculate a relative value to an average of an index from the index indicative of a state of an inspection object, which is calculated on a basis of a sensing signal; and a display processing section configured to perform processing to allow an image indicative of the state of the inspection object on a basis of the relative value to be displayed; characterized by: an average calculation section configured to calculate an average Na of the index according to Na = IRa − Ra IRa + Ra, which is obtained by averaging the index in an entire inspection object, wherein Ra denotes the average of all pixel values R of the red light and IRa denotes the average of all pixel values IR of the near-infrared light; and a correlation coefficient calculation section configured to calculate a correlation coefficient α that matches the average Na of the index with a predetermined specified value Nd, according to α = IRa 1 − Nd Ra 1 + Nd, wherein the relative value calculation section is further configured to apply the correlation coefficient α and to calculate the relative value Nr(x,y) of the index relative to the average Na of the index in each measurement unit (x,y) in which a measurement is performed on the inspection object, according to Nr x y = IR x y − αR x y IR x y + αR x y, on basis of a pixel value R(x,y) of red light and pixel value IR(x,y) of infrared light for the measurement unit (x,y). a relative value calculation section configured to calculate a relative value to an average of an index from the index indicative of a state of an inspection object, which is calculated on a basis of a sensing signal; and a display processing section configured to perform processing to allow an image indicative of the state of the inspection object on a basis of the relative value to be displayed; characterized by: an average calculation section configured to calculate an average Na of the index according to Na = IRa − Ra IRa + Ra, which is obtained by averaging the index in an entire inspection object, wherein Ra denotes the average of all pixel values R of the red light and IRa denotes the average of all pixel values IR of the near-infrared light; and a correlation coefficient calculation section configured to calculate a correlation coefficient α that matches the average Na of the index with a predetermined specified value Nd, according to α = IRa 1 − Nd Ra 1 + Nd, wherein the relative value calculation section is further configured to apply the correlation coefficient α and to calculate the relative value Nr(x,y) of the index relative to the average Na of the index in each measurement unit (x,y) in which a measurement is performed on the inspection object, according to Nr x y = IR x y − αR x y IR x y + αR x y, on basis of a pixel value R(x,y) of red light and pixel value IR(x,y) of infrared light for the measurement unit (x,y). an average calculation section configured to calculate an average Na of the index according to Na = IRa − Ra IRa + Ra, which is obtained by averaging the index in an entire inspection object, wherein Ra denotes the average of all pixel values R of the red light and IRa denotes the average of all pixel values IR of the near-infrared light; and a correlation coefficient calculation section configured to calculate a correlation coefficient α that matches the average Na of the index with a predetermined specified value Nd, according to α = IRa 1 − Nd Ra 1 + Nd, wherein the relative value calculation section is further configured to apply the correlation coefficient α and to calculate the relative value Nr(x,y) of the index relative to the average Na of the index in each measurement unit (x,y) in which a measurement is performed on the inspection object, according to Nr x y = IR x y − αR x y IR x y + αR x y, on basis of a pixel value R(x,y) of red light and pixel value IR(x,y) of infrared light for the measurement unit (x,y).