An optical fiber array substrate 11 has, in one surface thereof, eight V-grooves 12 for securing eight optical fibers 14 aligned in parallel to each other, and V-shaped side grooves 13 formed outside of the respective outermost V-grooves 12 located at the opposite sides of the substrate 11. The apexes of the outside ridges 12c and 12d defining the outermost V-grooves 12 are at the same height as the apexes of the inside ridges 12a and, the height of the bottom 13a of the side groove 13 is lower than that of a contact point 12e between the ridge line of the V-groove 12 and optical fiber 14.