A spot-size converter includes a cladding layer having a principal surface; a first core layer disposed on the principal surface and having a light input/output portion and a first transition portion having a width W1, the light input/output portion being coupled to the first transition portion and having a width that monotonously decreases in a first direction from the light input/output portion toward the first transition portion; and a second core layer disposed on the principal surface, the second core layer having a second transition portion and a propagation portion coupled to the second transition portion, the second transition portion having a width W2 . The first core layer has a refractive index between refractive indices of the second core layer and the cladding layer. The first transition portion and the second transition portion are disposed with a gap therebetween and optically coupled to each other. A ratio (W1/W2) of the width W1 to the width W2 monotonously decreases in the first direction.