Not Japan. The Iran nuclear deal was the G5+1, i.e., the UN Security Council countries plus German, so the US, the UK, France, Russia, and China plus Germany. None of the other countries have pulled out, but the fate of the deal is still up in the air because the US pulling out was a breaking of the deal and allows Iran to back out without repercussions, walking away with all the frozen assets in all those countries that were unfrozen in exchange. If and when we find out Iran has restarted its nuclear program, we will know that the US pulling out killed the deal.
It's basic contract law -- if any single signatory on one side of the contract falls out of compliance, it completely releases the signatory(s) on the other side of the contract from any and all contractual obligation, meaning when one party breaks a contract, it breaks the contract for everyone and so nobody else is obliged to comply with what they agreed to in that contract from that point forward.