A laser beam machining apparatus including a beam oscillator; a beam adjusting unit disposed between the beam oscillator and a condenser lens and by which the beam diameter and the divergence angle of the beam oscillated from the beam oscillator are adjusted; and a beam diameter and divergence angle detecting unit for detecting the beam diameter and the divergence angle of the beam having passed through the beam adjusting unit. The apparatus further includes a control unit which computes the beam diameter and the divergence angle of the beam, based on detection signals from the beam diameter and divergence angle detecting unit, and which controls the beam adjusting unit, based on the computed beam diameter and divergence angle of the beam and the beam diameter and the divergence angle of the laser beam incident on the beam diameter and divergence angle detecting unit.