A recording device records a video and an imaging time, and a voice. Based on the voice, a sound parameter calculator calculates a sound parameter for specifying magnitude of the voice in a monitoring area at the imaging time for each of pixels and for each of certain times. A sound parameter storage unit stores the sound parameter. A sound parameter display controller superimposes a voice heat map on a captured image of the monitoring area and displays the superimposed image on a monitor. At this time, the sound parameter display controller displays the voice heat map based on a cumulative time value of magnitude of the voice, according to designation of a time range.