After trying many of them, I think this one is the best Japanese restaurant in Madrid, at least the most authentic one (I guess other ones like Kabuki are slightly better but not as authentic).
It is located in a narrow street behind Gran Vía, if you are coming from Moncloa, for example, the best is to go up Leganitos and look for a parking place. Something important before going: Tuesday is supposedly the best day to go, you will get the freshest fish.
Even though we already know some unconditional fans of Miyama, we ordered the sample menu (38€ each). Well, before that, before we sat down even, I was surprised by the fact that there were two tables with Japanese people wearing suits. Japanese people in a Japanese restaurant in Madrid? Yes! This tells a lot about the place.