Biophoton emission is defined as extremely weak light that is radiated from any living system due to its metabolic activities, without excitation or enhancement. We measured biophoton images of tumors transplanted in mice with a highly sensitive and ultra-low noise CCD camera system. Cell lines employed for this study were AH109A, TE4 and TE9. Biophoton images of each tumor were measured 1 week after carcinoma cell transplantation to estimate the tumor size at week 1 and the biophoton intensity. Some were also measured at 2 and 3 weeks to compare the biophoton distribution with histological findings. We achieved sequential biophoton imaging during tumor growth for the first time. Comparison of microscopic findings and biophoton intensity suggested that the intensity of biophoton emission reflects the viability of the tumor tissue. The size at week 1 differed between cell lines, and the biophoton intensity of the tumor was correlated with the tumor size at week 1 (correlation coefficient 0.73). This non-invasive and simple technique has the potential to be used as an optical biopsy to detect tumor viability.