An image signal processing apparatus includes a clamp circuit that clamps an image signal having a horizontal synchronization signal, an optical black level period representing an optical black level, and an effective signal period representing an image signal for one horizontal line so as to clamp a value offset from the image signal on the basis of a first reference value during the optical black level period and to clamp the image signal on the basis of a second reference value different from the first reference value during the effective signal period, and a level computation circuit that determines the second reference value on the basis of a signal level clamped during the optical black level period.