For a pulsed high‐power laser with unstable resonator, the mode evolution time is important from the viewpoints of beam focusability and laser‐energy usefulness. We observed temporal and spatial behavior of the mode evolution in a several‐tens‐joule KrF laser with a positive branch confocal unstable resonator, which is pumped by electron beams with a pulse width of 65 ns (FWHM). The beam divergence angle was decreased by a factor of 1/M2(M:magnification) per round trip in the resonator and resulted in a full angle divergence of 0.6 mrad.