An optical guide to be used for detecting the remaining amount of developer contained in a developer containing portion includes a blocking portion for blocking an opening formed in the developer containing portion when the optical guide is fitted to the developer containing portion, an optical guide portion for guiding light, which is formed on a side to be positioned on the outside of the developer containing portion with respect to the blocking portion when the optical guide is fitted to the developer containing portion, an outside inclining surface inclining toward the side, on which the optical guide portion is formed, with respect to the blocking portion, the outside inclining surface being formed at an end portion of the optical guide portion on the blocking portion side in a lengthwise direction of the optical guide portion, and an inside inclining surface portion inclining toward a direction intersecting the lengthwise direction of the optical guide portion, the inside inclining surface portion being formed on a side to be positioned on an inside of the developer containing portion with respect to the blocking portion when the optical guide is fitted to the developer containing portion, wherein the optical guide is formed into one united body with a light-transmissive material.