A laser welding device is configured to switch an irradiation position of a measurement beam between a position of a keyhole coaxial with the optical axis of a laser beam and a position of a weld bead behind the center of an optical axis of the laser beam in a welding direction. The laser welding device determines whether there is a gap between an upper metal plate and a lower metal plate based on a measured value of a recess depth measured at the position of the weld bead.