A spot-size converter includes: a support body that includes a main surface including a first to a fifth areas; a mesa structure that includes a first part on the first area and includes a second part on the second to the fourth areas; and an embedding structure that includes a first region and a second region in which a first and a second side-surfaces of the second part of the mesa structure are respectively embedded. The second part of the mesa structure includes a portion that has a width gradually decreasing in a direction from the third area toward the fifth area. The first region of the embedding structure extends along the first side-surface and terminates at one of the third and the fourth areas. The second region of the embedding structure extends along the second side-surface of the second part and is disposed on the fifth area.