A theory is presented for the propagation of elastic waves in a plate excited by scanning a laser beam at a constant velocity. A formula for the excitation by scanning a line‐focused laser beam is deduced from a general formula and analyzed extensively in a two‐dimensional model. It is derived that such modes of Lamb waves are selectively excited whose phase velocities match the scanning velocity. It is also shown that major characteristics of experimental observations previously made are explained with the present model.