A disc apparatus comprising: a jitter-value detection unit configured to detect a jitter value based on a signal to be read from a medium; a defocus-value setting unit configured to set a defocus value for focusing an objective lens in the medium based on the jitter value; and a defocus-value adjusting unit configured to detect the jitter value every time the defocus value is changed stepwise within a predetermined range of the defocus value including a reference value of the defocus value, to obtain an optimum defocus value to be set for the defocus-value setting unit within a predetermined time period, based on a maximum jitter value and a minimum jitter value of the detected jitter values.