A focusing adjustment method includes calculating an in-focus level of an imaging optical system including a diaphragm and a focus lens based on an output signal of an image sensor, setting an optional focus lens position based on an instruction from a user, performing first control for determining a diaphragm driving direction and a diaphragm driving amount, based on the in-focus level; and performing second control for determining a moving direction of the focus lens, based on an output signal of the image sensor and determining a driving amount of the focus lens based on the diaphragm driving amount, wherein, in the first control and the second control, the diaphragm and the focus lens are controlled in such a range that a specific region of captured image can be included in depth of field.