In a solid state imaging device as an embodiment, an analog-to-digital converter unit converts, in a first period, a first pixel signal into a digital signal, performs, in a determination period after the first period, the comparison of a second pixel signal with the reference signal set to a predetermined threshold, and converts, in a second period after the determination period, the second pixel signal at a gain in accordance with a result of the comparison performed in the determination period into a digital signal. Until the reference signal reaches the threshold from the first period, the reference signal generation unit changes the reference signal without changing a direction of change of the reference signal with respect to the lapse of time.