Practice Relating to Nuclear Weapons
In its judgment in the Shimoda case in 1963, Japan’s District Court of Tokyo stated:
[I]t is right that the use of a new weapon is legal, as long as international law does not prohibit it. However, the prohibition in this case is understood to include … also the case where it is necessarily regarded that the use of a new weapon is prohibited, from the interpretation and analogical application of existing international laws and regulations (international customary laws and treaties). Further, we must understand that the prohibition includes also the case where, in the light of principles of international law which are the basis of the above-mentioned positive international laws and regulations, the use of a new weapon is admitted to be contrary to the principles.
The destructive power of the atomic bomb is tremendous … It is a deeply sorrowful reality that the atomic bombing on both cities of Hiroshima and Nagasaki took the lives of many civilians … In this sense, it is not too much to say that the pain brought by the atomic bombs is severer than that from poison and poison-gas, and we can say that the act of dropping such a cruel bomb is contrary to the fundamental principle of the laws of war that unnecessary pain must not be given. 
Japan, Shimoda et al., v The State, Tokyo District Court, 7 December 1963, 7 December 1963, as reproduced in International Law Reports, Vol. 32, pp. 626–642, at 628, 634.
In 1996, during a debate in the First Committee of the UN General Assembly, Japan stated:
Japan, which has experienced the calamity of atomic bombing, fervently hopes that nuclear weapons, which cause incomparable human suffering, will never again be used and firmly believes that continuous efforts should be made towards a nuclear-weapon-free world. Japan believes that because of nuclear weapons’ immense power to cause destruction, death and injury to human beings, their use is clearly contrary to the spirit of humanity that gives international law its philosophical foundation.
Japan firmly believes that we must take concrete measures to achieve steady progress in nuclear non-proliferation and nuclear disarmament. 
Japan, Statement before the First Committee of the UN General Assembly, UN Doc. A/C.1/51/PV.22, 14 November 1996, p. 7.