A laser welding apparatus includes: a first optical element that coaxially emits, to a welded part, a laser beam emitted from a laser light source and an object beam having a different wavelength from the laser beam; a second optical element that causes the spot diameter of the object beam to be larger than the spot diameter of the laser beam on the welded part; an optical interferometer that emits the object beam to the first optical element, detects through the first optical element the object beam reflected on the welded part, and generates an electric signal based on the detected object beam; and a measuring unit that measures a penetration depth of the welded part based on the electric signal.