When a video input (image data) sequentially fetched from a monitor camera is to be stored in data storage means, frame rate calculating means calculates a frame rate for storage of the image data by using a picture recording time mode, a residual capacity of the data storage means, the presence of generation of an alarm signal and a value of a picture quality parameter for data compression. The frame rate calculating means thins a video signal which does not correspond to the frame rate so as not to be stored in the data storage means. Consequently, image data for a predetermined time are stored in the data storage means.