The present technology relates to an AD conversion device, an AD conversion method, an image sensor, and an electronic apparatus that are able to achieve high-speed, low-power-consumption AD conversion. In a case where an electrical signal and a variable-level reference signal are compared by a comparator and the result of comparison is used to perform AD (Analog to Digital) conversion of the electrical signal, control is exercised in such a manner that a bias current flowing in the comparator to operate the comparator during a certain section of the reference signal including a section where the reference signal changes is increased from a first current, which is larger than 0 (zero), to a second current, which is larger than the first current. The present technology is applicable, for example, to AD conversion of an electrical signal.