A blood constituent concentration detector includes: a detection portion having an optical window for measuring a blood constituent concentration of a part of a living body in an optical manner when the part of the living body is pressed on the detection portion a load determination element for determining whether a load to the detection portion is equal to or larger than a first threshold, and smaller than a second threshold, wherein the second threshold is larger than the first threshold and a measurement element for measuring the blood constituent concentration when the load determination element determines that the load to the detection portion is equal to or larger than the first threshold, and smaller than the second threshold.
