A focal point detection device includes: a rate conversion section which converts a transmission rate of an image signal, which is associated with a subject image imaged via an imaging lens on a part of a receiving surface of an imaging device, such that the image signal input to the rate conversion section at a first transmission rate is output from the rate conversion section at a second transmission rate that is slower than the first transmission rate; a filter which extracts a frequency component from the image signal output at the second transmission rate from the rate conversion section; and an evaluation value calculation section which calculates an evaluation value of a focus state of the imaging lens, based on a signal of the frequency component that is extracted by the filter.