A system detects a candidate for an object which intrudes based on images of a visible light camera and a far-infrared camera that monitor the sea, further derives a size, a velocity, an intrusion direction, and linearity, and identifies the object to some extent. Also, the system distinguishes between a boat, a human, and a floating matter based on the luminances or the like in a far-infrared image. In addition, the system observes the periodicity of normal waves on the sea surface at a location without any object by performing the Fourier transform on the image. The accuracy of identification of an object is improved based on the correlation between the motion of waves in a normal state and the motion of the object.