To provide a connecting structure in a guide rail for a sliding door of a plastic greenhouse capable of certainly preventing deformation (bending) of the guide rail caused by weight of the sliding door arranged on the guide rail for the sliding door in a suspended state.

A connecting member 3 has a groove structure made of steel which opens downward and holds a peripheral surface of a sliding door guide rail 2. The connecting member 3 also comprises screw holes 40, 40 into which fixing bolts 4 are inserted on upper surface thereof and bolt holes 60, 70 which allow U-bolts 6 (7) pass through on common positions on both side surfaces. Each of end parts of the sliding door guide rails 2, 2 is introduced into the opening inner section of the connecting member 3 from right and left directions so that the end parts locate at the intermediate position of the connecting member 3 and contact each other. Tip ends of the fixing bolts 4 which are inserted into the screw holes 40, 40 are pressed onto the upper surface of the sliding door guide rail 2 so that the position of the sliding door guide rail 2 is fixed. The connecting member 3 is attached to a pillar member 13 with U-bolts 6 inserted into the bolt holes 60, 60.