A signal processing apparatus comprising: an analog-digital converter that uses reference signals having different slopes from each other; a supply unit that supplies analog signals of predetermined different output levels; and a calculation unit that calculates a slope ratio and an offset amount between the plurality of different slopes for correcting a converted digital signal based on digital signals respectively obtained by converting the analog signals of the different output levels using the reference signals. In a case where the conversion is performed using a reference signal with a steeper slope, the supply unit supplies at least one high level analog signal whose level is higher than a case where the conversion is performed using a reference signal with a gentler slope, and a digital signal obtained by converting the high level analog signal is further used when calculating the slope ratio.