Double beta decay is the slowest decay mode in nuclei and has a typical life time longer than 1019 years. This is a consequence of small Q values and tiny nuclear matrix elements, double Gamow-Teller matrix elements, linking the parent and grand-daughter nuclei. A special type of double beta decay, neutrinoless double beta decay, can be a direct evidence of Majorana nature of neutrinos and has been subject to world-wide efforts for the discovery. Once it is discovered, new properties of neutrinos will be extracted through the use of nuclear matrix elements. However, the nuclear matrix elements for the relevant transition, double Gamow-Teller transition, have been hardly investigated so far and its theoretical predictions are quite difficult. We started an experimental program to investigate double Gamow-Teller transitions in a wide excitation energy range with a newly invented experimental method to use the (12C,12Beγ) reaction. One of the highlight of the program is a discovery of the double Gamow-Teller giant resonances that exhaust a major part of the sum-rule value. The first experiment was carried out with a 100-MeV 12C beam at RCNP, Osaka University. We have found indication of the double Gamow-Teller giant resonances in 48Ca which is among double beta-decay nuclei. We plan high-statistics experiments with a high intensity 12C beam at RI Beam Factory(RIBF) which will be scheduled in 2019. In the colloquium, I will present results of the RCNP experiment and show future plans at RIBF, after brief introduction to double beta-decay physics and explanation of relevance of our experiments to the physics.