A spot size converter comprising: a first cladding layer; a first core layer and a second core layer arranged side by side on the first cladding layer so as to extend from a first end which receives/outputs light along a direction from the first end toward a second end; a -third core layer which is disposed on the first cladding layer between the first and second core layers (13, 14), is a member different from the first and second core layers (13, 14) that is disposed apart from the first and second core layers (13, 14) in a direction orthogonal to a stack direction of the layers and orthogonal to the direction from the first end toward a second end, but is optically coupled to the first and second core layers (13, 14), and extends to the second end along the direction from the first end toward the second end; and a second cladding layer disposed on the first, second, and third core layers (13, 14, 15) wherein the sectional areas of the first and second core layers (13, 14) increase in a direction from the first end toward the second end, reaches the maximum value and, after that, decreases, the third core layer is disposed so as to extend from a position in the parts (13a, 14b) where the sectional area of the first and second core layers (13, 14) is the maximum to the second end in the direction from the first end toward the second end, a sectional area of the third core layer increases in a direction from the first end toward the second end, and wherein the sectional area of the third core layer is the maximum in a position where sectional areas of the first and second core layers (13,14) are the minimum in the direction from the first end toward the second end. a first cladding layer; a first core layer and a second core layer arranged side by side on the first cladding layer so as to extend from a first end which receives/outputs light along a direction from the first end toward a second end; a -third core layer which is disposed on the first cladding layer between the first and second core layers (13, 14), is a member different from the first and second core layers (13, 14) that is disposed apart from the first and second core layers (13, 14) in a direction orthogonal to a stack direction of the layers and orthogonal to the direction from the first end toward a second end, but is optically coupled to the first and second core layers (13, 14), and extends to the second end along the direction from the first end toward the second end; and a second cladding layer disposed on the first, second, and third core layers (13, 14, 15) wherein the sectional areas of the first and second core layers (13, 14) increase in a direction from the first end toward the second end, reaches the maximum value and, after that, decreases, the third core layer is disposed so as to extend from a position in the parts (13a, 14b) where the sectional area of the first and second core layers (13, 14) is the maximum to the second end in the direction from the first end toward the second end, a sectional area of the third core layer increases in a direction from the first end toward the second end, and wherein the sectional area of the third core layer is the maximum in a position where sectional areas of the first and second core layers (13,14) are the minimum in the direction from the first end toward the second end.