© 2017 Elsevier B.V. A multipixel photon counter (MPPC) features excellent photon-counting capability as a radiation detector. In particular, a two-plane Compton camera consisting of Ce:GAGG scintillators coupled with MPPC arrays has significant application potential owing to its compact size and low weight. For example, the camera can be easily mounted on a commercial drone to identify radiation hot spots from the sky. In Fukushima, we demonstrated that a 137Cs distribution within a 100 m diameter can be mapped correctly within a couple of tens of minutes. The advanced use of the Compton camera is also anticipated in the field of proton therapy. We evaluated an image of 511 keV annihilation gamma-rays emitted from a PMMA phantom irradiated by 200 MeV protons to mimic an in-beam monitor for proton therapy. Finally, we developed an ultracompact Compton camera (weight = 580 g), for 3-D multicolor molecular imaging. In order to demonstrate the performance capabilities of the device, 131I (365 keV), 85SrCl2 (514 keV), and 65ZnCl2 (1116 keV) were injected into a living mouse and the data were taken from 12 angles with a total acquisition time of 2 h. We confirmed that all tracers had accumulated on the target organs of the thyroid, bone, and liver, and that the obtained 3-D image was quantitatively correct with an accuracy of ±20%.