An image reading signal processing IC to which analog image signals are input, where the analog image signals are formed by optically reading color image information of a document and converting the read color image information into electric signals corresponding to three primary colors, the image reading signal processing IC (10, 20) characterized by: three circuits which independently process the input analog image signals of the corresponding three primary colors; characterized in that each of the three circuits includes a sample-hold circuit which samples the analog image signals and holds the values of the sampled analog image signals; a VGA (variable gain amplifier) which amplifies the analog image signals held by and output from the sample-hold circuit; an ADC (analog to digital converter) which converts the amplified analog image signals into digital image data; an LVDS (low voltage differential signaling) processing circuit which outputs the digital image data in a low voltage differential signaling level; a test pattern generating circuit connected between the ADC and the LVDS processing circuit for generating digital data of a test pattern; and the image reading signal processing IC (10, 20) further characterized by; an operational mode determination information input unit (18, 23) which inputs operational mode determination information to the test pattern generating circuit; characterized in that an operational mode of each of the test pattern generating circuits is switched between a normal mode which outputs digital image data output from the ADC and a test pattern output mode which outputs the digital data of the test pattern generated by the test pattern generating circuit corresponding to the operational mode determination information input from the operational mode determination information input unit (18, 23). three circuits which independently process the input analog image signals of the corresponding three primary colors; characterized in that each of the three circuits includes a sample-hold circuit which samples the analog image signals and holds the values of the sampled analog image signals; a VGA (variable gain amplifier) which amplifies the analog image signals held by and output from the sample-hold circuit; an ADC (analog to digital converter) which converts the amplified analog image signals into digital image data; an LVDS (low voltage differential signaling) processing circuit which outputs the digital image data in a low voltage differential signaling level; a test pattern generating circuit connected between the ADC and the LVDS processing circuit for generating digital data of a test pattern; and the image reading signal processing IC (10, 20) further characterized by; an operational mode determination information input unit (18, 23) which inputs operational mode determination information to the test pattern generating circuit; an operational mode of each of the test pattern generating circuits is switched between a normal mode which outputs digital image data output from the ADC and a test pattern output mode which outputs the digital data of the test pattern generated by the test pattern generating circuit corresponding to the operational mode determination information input from the operational mode determination information input unit (18, 23).