A contact-type linear sensor using amorphous Si diodes as photodetectors has been developed. The sensor structure can largely reduce the number of electrodes to be connected to the switching FETs and hence has an advantage of realizing a facsimile reader which is compact and easy to construct. The linear sensor developed is constructed with 256 pairs of In2O3/a-Si photodiodes and Pt/a-Si blocking diodes and this combination of two types of diodes has been found to yield high output signal. In this sensor structure, incident light is introduced to the photodiodes through a glass substrate in order to protect the photodetectors from contamination and scratching.