A biological information monitoring apparatus highly precisely observes a clothed subject who is taking any desired posture. The apparatus acquires information on positional displacements of the surface of a living body, by generating a high-frequency electromagnetic wave to radiate it into air, detecting the electromagnetic wave scattered by the surface of the living body, computationally determining the changes with time of the positional displacements of the surface of the living body from the state of propagation of said electromagnetic wave and computationally determining characteristic quantities of vibrations of the living body such as the pulse or the respiration from the changes with time as biological information.