A scanner device can be used in a facsimile machine or the like for reading a document on line basis. Each line of the document is read on pixel basis while irradiating a light onto the document. The scanner device includes an image sensor which receives a light reflected from the document and converts the light to an analog electrical signal. A controller controls the reading operation of the image sensor. A comparator translates the analog electric signal into a binary signal. The binary signal is stored in a shift register wherein the contents therein represents an image of a preceding line with respect to the present line being read by the image sensor. A threshold value applied to the comparator to read the present line image is varied depending upon the preceding line data stored in the shift register. As such, images on the document can be read with excellent fidelity, particularly when a fine line extending in a main scanning direction is present on the document.