Terahertz (THz) technology is a focus of attention in research on applied optics. We have applied THz spectroscopy and THz-imaging method to text recognition of a medieval manuscript made from sheepskin. Based on the database which contains more than 200 spectra of art materials, the red ink on the manuscript was estimated as Cinnabar. The red ink text and stains on the text were successfully distinguished by the THz-Time Domain Spectroscopy (TDS) imaging with a component spatial pattern analysis. THz spectroscopy can be used as a non-invasive analysis method for conservation science of cultural properties.