PURPOSE: To make it possible to execute trial writing in a place where information is actually recorded or near this place without requiring an excessive power and to set the power at the optimum recording power.

CONSTITUTION: The recording powder is set at the pulse width τ1 longer than the prescribed pulse width τ0 prior to recording of information in the method for determining the recording power which determines the optimum recording power of a laser beam. The optimum recording power P1 is scanned and found by executing trial writing with the plural recording powders and thereafter the optimum power PO at the prescribed pulse width τ0 is determined from the pulse width τ1 and the recording power P1. The actual measurement for determining the optimum powder at the prescribed pulse width τ0 is not executed.