An optical fiber with a lens, obtained by fusion-coupling a multi-mode optical fiber to a tip of a single-mode optical fiber or polarization maintaining optical fiber, and working the tip of the multi-mode optical fiber, to form a spherical portion at the tip. In this case, the radius of curvature of the spherical portion is set to be larger than the maximum value of the expanded spot size. Furthermore, the length of the multi-mode optical fiber is set based on the correlation depending on said length and said radius of curvature for obtaining the minimum spot size at the focal position.