In a sensor device which detects pressure variations owing to displacements of a body surface of a subject by respiration through variations of the output impedance of a piezoelectric vibrator, the pressure variations owing to displacements can be more efficiently transmitted to the piezoelectric vibrator. The sensor device, attached to a subjects body surface and detecting displacements of the body surface accompanying respiration of the subject, comprises a contact piece having a contact surface which contacts the body surface when the sensor device is attached to the body surface a displacement-signal convertor which is connected with the contact piece and converts the displacements of the body surface into electrical signals and an outputting device which outputs the electrical signals to an external unit wherein the contact surface of the contact piece is a curved surface projecting outward in a direction from its periphery to its center.
