A laser unit having a preparatory function for activating the unit and an activation method for the unit, capable of stabilizing the temperature of each component of the laser unit in the preparation stage when the laser unit is activated in a low-temperature or high-temperature environment, and then effectively judging completion of the preparation. During a period of time Ta from T1 to T2, a first trial of laser oscillation is performed. After a period of time Tb in which the laser is not oscillated, a second trial of laser oscillation from T3 to T4. Such an operation is repeated until a predetermined criterion is satisfied. Laser output values P2, P4, P6, . . . , at the last moment in each trial of laser oscillation are recorded. Then, each difference between the laser outputs at the last moments of two neighboring or continuous trials, is calculated. When the difference is lower than a predetermined criterion value, the preparation of the laser unit is judged to be completed.