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The composition of cellulosomal carbohydrate-active enzymes (CAZymes) secreted from a cellulolytic bacterium Clostridium thermocellum varies depending on the cellulosic substrate used during cultivation. C. thermocellum detects the polysaccharides in cellulosic material via anti-sigma factors and expresses the appropriate CAZyme gene via alternative sigma factors, SigIs. Previous studies on the regulation of CAZyme gene expression via SigIs in C. thermocellum have been conducted in vitro or in a heterologous host, because of the limited genetic tools available for C. thermocellum. To characterize the in vivo function of SigIs, in the present study, we established a sigI7 gene expression strain of C. thermocellum. Transcriptome analysis of this strain revealed that SigI7 induced the expression of cellulosomal CAZyme genes and cellulosomal scaffold genes. However, there was a decrease in the degradation ability of the exoproteome from the sigI7 expression strain; the product of the downregulated gene, Clo1313_1002, rescued the activity of the C. thermocellum exoproteome from the sigI7 expression strain. In this study, we demonstrate the in vivo function of SigI7 and discuss the CAZymes that are important for cellulosic biomass degradation by C. thermocellum.