A parallel-beams type collector comprises a flat porous platform which is enclosed by side walls. The flat porous platform is formed from a row of horizontally projected, parallel and elongated tines or thin rods. The debris is moved onto the porous platform as the collector is moved forward across the lawn with the elongated thin rods moving and penetrating through the grass. The collector is moved up along a curved path by a moving assembly. A flexible string is pulled via an array of rollers to move the collector up. The leaves and debris sitting in the collector can further be swept away by a sweeping assembly. Alternatively, a continuous endless chain is used to move the collector. A guide member is used to keep the collector in proper orientation during the collection of the debris.