To reduce the effect of a voltage drop in each optical sensor element as the loss of incident light due to free-carrier absorption is kept low in a quantum well optical sensor and a method of manufacturing the quantum well optical sensor.

A multitude of optical sensor elements 5, which are subjected to photodetection utilizing light absorption using a carrier excitation between subbands in multiple quantum well layers 6, are arranged in an array type and at the same time, a carrier concentration in a lower contact layer 2 provided under the lower parts of the elements 5 is partially made to differ. That is, as the regions, which are overlapped with the elements 5 in a projective manner, of the layer 2 are provided as low-carrier concentration regions and the peripheries of the low-carrier concentration regions are provided as high-carrier concentration regions, reduction in an element sensitivity, which is felt as the whole quantum well optical sensor, due to free-carrier absorption is never caused and the effect of a voltage drop in each optical sensor element can be reduced. As a result, the fluctuation of a bias, which is applied to each optical sensor element, and an irregularity in the bias can be lessened, whereby the quantum well optical sensor greatly contributes to the practical use of a large-area, high-integration degree and high-resolution infrared sensor array.