A laser crystallizing apparatus includes a first light source unit configured to emit a first input light having a linearly polarized laser beam shape. A second light source unit is configured to emit a second input light having a linearly polarized laser beam shape. A polarization optical system is configured to rotate the first input light and/or the second input light at a predetermined rotation angle. An optical system is configured to convert the first input light and the second input light, which pass through the polarization optical system, into an output light. A target substrate is seated on a stage and output light is directed onto the target substrate. A monitoring unit is configured to receive the first input light or the second input light from the polarization optical system and measure a laser beam quality thereof.