An image pickup apparatus arranged to detachably mount a lens unit having an optical system including a focus lens, such that communication is enabled between the image pickup apparatus and the lens unit when the lens unit is mounted to the image pickup apparatus, the image pickup apparatus comprising: image pickup means arranged to accumulate charges by detecting light which passed through the optical system of the attached lens unit, and periodically generating an image signal from the charges with a predetermined cyclical period; extraction means arranged to extract a focus signal from the image signal corresponding to a focus detection area in an image pickup plane of the image pickup means; and control means arranged to set first information relating to an amount of movement of the focus lens on the basis of the focus signal extracted by the extraction means, and to transmit the first information to the lens unit, and to receive information, synchronously with the predetermined cyclical period, on the basis of which the control means is arranged to determine whether the focus lens is movable by the amount of movement corresponding to the first information within a predetermined time, from the lens unit, wherein if the control means determines that the focus lens is movable by the amount of movement corresponding to the first information within the predetermined time based on the information received from the lens unit, the control means decides a next movement amount of the focus lens on the basis of a first focus signal extracted from the image signal corresponding to the charges accumulated in a first period, and if the control means determines that the focus lens is not movable by said amount of movement corresponding to the first information within the predetermined time based on the information received from the lens unit, the control means decides a next amount of movement of the focus lens, without using the first focus signal, on the basis of a second focus signal extracted from the image signal corresponding to the charges accumulated in a second period after the first period and after the movement of the focus lens corresponding to the first information is completed so that the movement of the lens is stopped. image pickup means arranged to accumulate charges by detecting light which passed through the optical system of the attached lens unit, and periodically generating an image signal from the charges with a predetermined cyclical period; extraction means arranged to extract a focus signal from the image signal corresponding to a focus detection area in an image pickup plane of the image pickup means; and control means arranged to set first information relating to an amount of movement of the focus lens on the basis of the focus signal extracted by the extraction means, and to transmit the first information to the lens unit, and to receive information, synchronously with the predetermined cyclical period, on the basis of which the control means is arranged to determine whether the focus lens is movable by the amount of movement corresponding to the first information within a predetermined time, from the lens unit, wherein if the control means determines that the focus lens is movable by the amount of movement corresponding to the first information within the predetermined time based on the information received from the lens unit, the control means decides a next movement amount of the focus lens on the basis of a first focus signal extracted from the image signal corresponding to the charges accumulated in a first period, and if the control means determines that the focus lens is not movable by said amount of movement corresponding to the first information within the predetermined time based on the information received from the lens unit, the control means decides a next amount of movement of the focus lens, without using the first focus signal, on the basis of a second focus signal extracted from the image signal corresponding to the charges accumulated in a second period after the first period and after the movement of the focus lens corresponding to the first information is completed so that the movement of the lens is stopped.