An optical fiber connection structure includes a single-core plug holding a single-core ferrule, a multi-core plug holding a multi-core ferrule, and an adaptor having a tubular sleeve. The single-core ferrule made of zirconia (ZrO2) has a cylindrical shape and is held in a holder made of stainless steel (SUS). The single-core ferrule has a convex end surface whose curvature radius R1 is at least 50 mm. The multi-core ferrule made of zirconia (ZrO2) has a cylindrical shape and is held in a holder made of stainless steel (SUS). The multi-core ferrule has a convex end surface whose curvature radius R2 is at least 18.3 mm and at most 38.7 mm. When the first and second plugs are attached to the adaptor, the singe-core ferrule and the multi-core ferrule are respectively inserted from the opposite ends into the sleeve, and both of the end surfaces of the ferrules are pressed against each other.