An object information acquiring apparatus is used which includes: a laser medium; a temperature controlling unit for controlling a temperature of the laser medium; an excitation unit for exciting the laser medium; a control unit for controlling the temperature controlling unit and the excitation unit; a probe for receiving acoustic waves that are generated when an object is irradiated with a laser beam from the laser medium; and a processing unit for acquiring characteristic information relating to the object from the acoustic waves, wherein the control unit does not operate the excitation unit until the temperature of the laser medium falls within a predetermined range.