Optical Device, Optical Module, and Method for Manufacturing Optical Device or Optical Module

A technology with a simpler and smaller configuration and capable of suppressing light leakage from an optical module is provided. An optical device (3) includes an optical region (3a) in which an optical element is arranged, at a part of at least one surface of a substrate portion (3b) having a flat plate shape. A peripheral region (3d) in which an optical function as an optical device is not used is provided around the optical region (3a) at the surface, of the substrate portion (3b), provided with the optical region (3a). A light-blocking and diffusing portion (3e) configured to suppress transmission of light or diffuse light is formed at the peripheral region (3d) or a region corresponding to the peripheral region (3d) at an opposite surface of the substrate portion (3b).