Abstract: A reframer and Loss Of Frame (LOF) checking apparatus excludes an input frame data having an improper start point, carries out a reframing process, checks whether the reframed data is normal, and outputs a LOF state to a checking board or operator, in accordance with a predetermined Frame Alignment Signal (FAS) of a digital hierarchy structure. The reframer and Loss Of Frame (LOF) checking apparatus includes frame filtering and realigning units for detecting a frame start point of input frame data by using the FAS, excluding input data having an improper start point that is caused by a frame data loss, by using the frame start point detecting value, and outputting reframed data having a normal frame format. The apparatus also includes LOF declaring and releasing units for re-detecting the FAS in the reframed data, checking whether the reframed data is normal, setting a frame loss or loss releasing state indicator, according to the checking result, and outputting each state indicator signal.