Common marmosets (Callithrix jacchus) have been used as primate model to investigate the neural basis of auditory information processing. Although neuronal representations and coding mechanisms of sound in marmosets have been revealed using single- and multi-unit recording methods, it is difficult to predetermine effectively the region of interest because most cerebral sulci of marmosets are too shallow to use as reliable cortical landmark. Optical imaging methods are useful tools for visualizing spatial organizations of a neural architecture to overcome the difficulty. Here we show the tonotopy map on the primary auditory cortex of an anesthetized marmoset mapped by optical intrinsic signal imaging (OISI). We presented pure tones and broadband noises ranging from 500 to 12 kHz in 0.5 octave steps. The bandwidth of each broadband noise was 0.5 octaves. We illuminated cortical surface using band-passed light with 535 nm center wavelength. Changes in backscattered light following a stimulus were acquired as evoked optical signals. We were able to identify the auditory cortex as an area that the pre tones and broadband noises selectively evoked the negative changes in the optical signals. The stimulus responsive regions were tonotopically organized; preferred stimulus frequency decreased from low to high in caudal to rostral direction. The responsive regions to each pure tone overlapped with those to a broadband noise with its center frequency matched. A following electrophysiological recording confirmed that the responsive regions revealed by OISI contained cells that strongly responded to the same stimuli. The stimulus selectivity of the cells were well correlated with that of the optical signal changes at the corresponding recording sites (averaged correlation coefficients: 0.67, n=32). These results indicate that combination of OISI and single- or multi-unit recording applied to the lssencephalic cortex of marmosets will widen the experimental approaches to elucidate auditory information processing in the primates.