A sub-mount having a photodiode region, includes a photodiode which has a first conductivity-type layer arranged in a surface portion of the sub-mount of the photodiode region to form a light-receiving surface and a second conductivity-type region arranged below the first conductivity-type layer and is configured to receive at the light-receiving surface a light emitted from a light-emitting element and convert the light into a photocurrent. A peak light-receiving wavelength at which the photocurrent of the photodiode becomes its maximum value is more than or equal to a minimum emission wavelength of the light-emitting element and less than or equal to a maximum emission wavelength of the light-emitting element.