PURPOSE: To increase a transmission band numerical aperture of a multi-mode waveguide by forming the first layer (refractive index n1) of the center through the fourth layer to a concentric circle shape, setting a refractive index of each layer as n1>n2>n4>n3, and using the first layer and the second layer as a single mode waveguide and a multi-mode waveguide, respectively.

CONSTITUTION: An optical fiber 5 is constituted of a four layer sectional structure in which the second layer 2, the third layer 3 and the fourth layer 4 (refractive indexes n2, n3 and n4) have been formed in a concentric circle shape toward the outside of the diameter direction from the first layer (refractive index n1) of a circular section of the center. The large and small relation of the refractive index of each layer is as shown by n1>n2>n4>n3, and the first layer 1 and the second layer 2 are used as a single mode waveguide and a multi-mode waveguide, respectively.