In an object detection system with a first and a second image processing apparatus, the first image processing apparatus includes a reduction unit configured to reduce an input image, a first detection unit configured to detect a predetermined object from a reduction image reduced by the reduction unit, and a transmission unit configured to transmit the input image and a first detection result detected by the first detection unit to the second image processing apparatus, and the second image processing apparatus includes a reception unit configured to receive the input image and the first detection result from the first image processing apparatus, a second detection unit configured to detect the predetermined object from the input image, and an output unit configured to output the first detection result and a second detection result detected by the second detection unit.