The present invention relates to a GI-type multi-mode optical fiber in which the outer diameter 2a of a core is 47.5 to 52.5 μm or 60 to 65 μm. In the multi-mode optical fiber, stress in the optical axis direction remaining in an outermost peripheral portion of the cladding is tensile stress of 0 to 25 MPa, the outermost peripheral portion of the cladding being defined as a region having a diameter of 1.8b or more when the diameter of the cladding is 2b.