Provided is a gain-clamped optical amplifier amplifying light without bandwidth loss of an incident optical signal, and the gain-clamped optical amplifier using a double-clad fiber comprises: an optical fiber including a core doped with a gain material for amplifying an optical signal, a primary clad adjacent to the outside of the core and having a lower refractive index than the core, and a secondary clad adjacent to the outside of the primary clad and having a lower refractive index than the primary clad; a light emitting element emitting a pump light for population inversion of the gain material; and a cavity unit producing laser oscillation by resonating spontaneous emission light emitted from the gain material population-inverted by the pump light.