An image forming apparatus comprises image forming stations Y, M, C, K for respective colors arranged along a transfer belt 16, each image forming station including an image carrier 20, a charging means 22 and a developing means 24 disposed around the image carrier 20. The longitudinal part is a metallic blade where the inertias on the two axes of its section are very different. Pairs of coincidence gamma rays emitted from positron annihilation events are recorded by the pair of gamma detectors.