A pressure-sensitive sensor (a pressure sensor) for measuring the pressure applied to a pressure-receiving part by assuming the pressure as an amount of deformation of the pressure-receiving part and converting the amount of deformation into an electrical signal and a monitor for monitoring the action and the state and change of the emotional motion of a person through the pressure-sensitive sensor, for example, a pet robot that senses the petting action of the owner and performs a predetermined reaction and action are disclosed. The sensor comprises at least two parallel electrode sheets disposed in a spaced relationship from each other and an expandable dielectric member interposed between the adjacent two electrode sheets. When a pressure is exerted on the outermost electrode sheet, the dielectric member deforms and its capacitance varies.