NOVELTY - The organic waste composter (1) comprises a reservoir tank (10), composting tanks (20, 21, 22) positioned above the reservoir tank, a fixing unit, and a lid pivotally mounted above the composting tank located in an uppermost position with respect to the reservoir tank. The reservoir tank comprises: a rigid bottom having a groove that is positioned on an external face of the bottom and is vertical with respect to an operating position for receiving a part of a guardrail to fit the composter over the guardrail; and a tray (13) for collecting liquid produced by the composter.    USE - The organic waste composter is useful for an apartment, and for recycling waste of organic material by decomposing.    ADVANTAGE - The organic waste composter avoids the presence of flies and problem of congestion in the apartment, and allows complete recycle of waste without unpleasant odor.    DETAILED DESCRIPTION - The organic waste composter (1) comprises a reservoir tank (10), composting tanks (20, 21, 22) positioned above the reservoir tank, a fixing unit, and a lid pivotally mounted above the composting tank located in an uppermost position with respect to the reservoir tank. The reservoir tank comprises: a rigid bottom having a groove that is positioned on an external face of the bottom and is vertical with respect to an operating position for receiving a part of a guardrail to fit the composter over the guardrail; a tray (13) for collecting liquid produced by the composter; a locating portion of the composting tank disposed in the groove for receiving the guardrail; and a gardening portion (12). The fixing unit is adapted for fixing the composter on the guardrail, is integral with the groove, and comprises a U-shaped section integral with the groove, a unit for clamping the U-shaped section against the guardrails and two pieces forming two clamping jaws. The lid has a slope. The U-shaped section comprises: two through slots intended for receiving a clamping collar for fixing the U-shaped section against the guardrail; two through holes intended for passage of two bolts; a flange secured to a spacer with the U-shaped section; a clamping plate intended to come into tight contact with the guardrail such that the space tends to press the clamping plate against the guard body; and a hole in which passes a rod slidably mounted in an oblong slot of two parts mounted opposite with respect to each other to clamp the guard rail between the two parts. The clamping collar is made of metal or polymer. The two clamping jaws are intended to be clamped about a portion of the guardrails by screwing both bolts. The spacer comprises a resilient unit for allowing automatic clamping. The resilient unit comprises a spring or a prestressed blade. The U-shaped section further has spacing between the free ends of the flange slightly greater than a width of the groove to be inserted and held by force into the groove. The groove has complementary parts such as a projection and a slot for allowing the locking of the section in the groove. The composting tank is mounted on a sliding support disposed above the reservoir tank. An independent claim is included for a unit for fixing a composter on a guardrail.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a composter.    Composter (1)    Reservoir tank (10)    Gardening part (12)    Liquid collecting tray (13)    Composting tanks. (20, 21, 22)