A signal processing device (1, 20D) including: a switching unit configured to be switched between a switching mode and a non-switching mode; wherein, in the switching mode, the switching unit is configured to perform, in response to a comparison between an indication value indicating the brightness of a captured image obtained by an imaging device and a threshold value (F_th), a switching control consisting of switching between an optical brightness adjustment being an iris adjustment depending on the indication value and an electronic brightness adjustment being an application of a gain depending on the indication value, wherein the indication is an F target value; and wherein, in the non-switching mode, the switching unit is configured to not perform the switching control with respect to the comparison between the indication value and the threshold value, but to execute the optical brightness adjustment according to an F target value indicated by an operation of an adjustment operator of a remote controller, and a first delay unit (7b, 8) configured to, in the switching mode, delay a change in the gain depending on the indication value in the electronic brightness adjustment. a switching unit configured to be switched between a switching mode and a non-switching mode; wherein, in the switching mode, the switching unit is configured to perform, in response to a comparison between an indication value indicating the brightness of a captured image obtained by an imaging device and a threshold value (F_th), a switching control consisting of switching between an optical brightness adjustment being an iris adjustment depending on the indication value and an electronic brightness adjustment being an application of a gain depending on the indication value, wherein the indication is an F target value; and wherein, in the non-switching mode, the switching unit is configured to not perform the switching control with respect to the comparison between the indication value and the threshold value, but to execute the optical brightness adjustment according to an F target value indicated by an operation of an adjustment operator of a remote controller, and wherein, in the switching mode, the switching unit is configured to perform, in response to a comparison between an indication value indicating the brightness of a captured image obtained by an imaging device and a threshold value (F_th), a switching control consisting of switching between an optical brightness adjustment being an iris adjustment depending on the indication value and an electronic brightness adjustment being an application of a gain depending on the indication value, wherein the indication is an F target value; and wherein, in the non-switching mode, the switching unit is configured to not perform the switching control with respect to the comparison between the indication value and the threshold value, but to execute the optical brightness adjustment according to an F target value indicated by an operation of an adjustment operator of a remote controller, and a first delay unit (7b, 8) configured to, in the switching mode, delay a change in the gain depending on the indication value in the electronic brightness adjustment.