A method for refining a magnetic domain of a grain-oriented electrical steel strip is provided, including a steel strip supporting roll position adjusting step of controlling a position of the steel strip in a vertical direction while supporting the steel strip proceeding along a production line, a laser irradiating step of forming a groove on a surface of the steel strip by irradiating a laser beam onto the surface of the steel strip to melt the steel strip, and a detecting step of detecting a defect in the groove formed on the surface of the steel strip while the steel strip proceeds, so as to be able to detect whether the groove is defective by confirming a machining state of a magnetic domain refined groove formed on the surface of the steel strip in a working process.