A single-mode optical fiber for outputting high-power light wherein a diameter of a mode field at an output end of the optical fiber is made large to thereby lower a light power density. Material higher in thermal conductivity or melting temperature than material of the optical fiber is coated on the end face of the fiber. A thickness of the coating film is set to cause light wavelength to be phase-shifted by 1/4 of the wavelength to reduce reflection at the connecting end face. The same type of optical fiber is used as a party optical fiber to be connected therewith.