To provide a monitoring device which can detect conditions of a sleeping person reliably and which is simple. A monitoring device comprising: multiple independent distance sensors 11 installed facing different positions in a monitored target area 50 to be monitored for measuring a distance to a monitored target 2, a calculating unit 22 for calculating changes over time in the outputs of the distance sensors 11, and a detection processor 23 for detecting changes in shape of the monitored target 2 based on the calculated changes over time in one or multiple distance sensor 11 among the multiple distance sensors 11.