1. A detecting apparatus for detecting a motion state of a person to be observed from an image captured, comprising: an input/output unit; a memory for storing video data which is received by the input/output unit; and a control unit controlling a process of detecting a motion state of the person to be observed on the basis of the video data stored in the memory, wherein the control unit comprises: a calculating unit calculating a motion vector of each of a plurality of blocks of an image including the video data; a detecting unit extracting a block group as a set of the blocks in each of which the size of the motion vector exceeds a predetermined value on the basis of the calculated motion vectors and detecting an abnormal motion state of the person to be observed on the basis of the motion vectors of the blocks included in the extracted block group, and an informing control unit outputting a signal indicative of an abnormal state of the person to be observed in the case where the abnormal motion state of the person to be observed is detected by the detecting unit.