A silica glass base material for forming an optical fiber comprising: a core with a radius to r 1 at a central portion of the silica glass base material; an intermediate layer with an outermost radius to r 2 that is adjacent to the core at a radial position ri and covers a periphery of the core; a low refractive index trench layer with an outermost radius to r 3 that is adjacent to the intermediate layer at a radial position r 2 and covers a periphery of the intermediate layer; and an outer cladding layer that is adjacent to the low refractive index trench layer at a radial position r 3 and covers a periphery of the low refractive index trench, wherein a refractive index of the intermediate layer decreases smoothly and continuously from an inside to an outside thereof, has a maximum value at the radial position r 1 , and has a minimum value at the radial position r 2 , wherein the refractive index profile of the intermediate layer has a protruding portion in which the second derivative of the refractive index changes from positive to negative, and then to positive, from the core side to the low refractive index trench layer side, wherein the core has a maximum refractive index n 0 , the intermediate layer has a refractive index n 1 at the radial position r 1 and a refractive index n 2 at the radial position r 2 , the low refractive index trench layer has a minimum refractive index n 3 , the outer cladding layer has a minimum refractive index n 4 , and n 0 > n 1 > n 2 > n 3 and n 3 < n 4 , and wherein in a region outside the radial position r 2 at which the intermediate layer contacts the low refractive index trench layer, there is an inflection point in a curve representing a refractive index profile, characterized in that the silica glass base material includes OH groups at an interface between the intermediate layer and the low refractive index trench layer at 0.5 ppm to 5 ppm by weight. a core with a radius to r 1 at a central portion of the silica glass base material; an intermediate layer with an outermost radius to r 2 that is adjacent to the core at a radial position ri and covers a periphery of the core; a low refractive index trench layer with an outermost radius to r 3 that is adjacent to the intermediate layer at a radial position r 2 and covers a periphery of the intermediate layer; and an outer cladding layer that is adjacent to the low refractive index trench layer at a radial position r 3 and covers a periphery of the low refractive index trench, wherein a refractive index of the intermediate layer decreases smoothly and continuously from an inside to an outside thereof, has a maximum value at the radial position r 1 , and has a minimum value at the radial position r 2 , wherein the refractive index profile of the intermediate layer has a protruding portion in which the second derivative of the refractive index changes from positive to negative, and then to positive, from the core side to the low refractive index trench layer side, wherein the core has a maximum refractive index n 0 , the intermediate layer has a refractive index n 1 at the radial position r 1 and a refractive index n 2 at the radial position r 2 , the low refractive index trench layer has a minimum refractive index n 3 , the outer cladding layer has a minimum refractive index n 4 , and n 0 > n 1 > n 2 > n 3 and n 3 < n 4 , and wherein in a region outside the radial position r 2 at which the intermediate layer contacts the low refractive index trench layer, there is an inflection point in a curve representing a refractive index profile, characterized in that the silica glass base material includes OH groups at an interface between the intermediate layer and the low refractive index trench layer at 0.5 ppm to 5 ppm by weight. <EOS>