The present invention relates to an optical fiber for amplification, an optical amplifying apparatus, and so on, having a structure for enabling increase of power of output light, suppression of occurrence of nonlinear optical phenomena, and compact storage all together. The optical amplifying apparatus comprises an optical fiber for amplification amplifying input light inputted together with pumping light from a pumping light source. The optical fiber for amplification comprises a core region doped with a rare-earth element, and a cladding region provided on an outer periphery of the core region and having a refractive index lower than that of the core region. The core region has an outer diameter of 10 .mu.m or more but 30 .mu.m or less, and has a relative refractive index difference of 0.5% or more but 2.0% or less with respect to the cladding region. The cladding region has an outer diameter of 75 .mu.m or more but 200 .mu.m or less.