Should I bring my own ski gear to Japan, or rent there?
At the bigger international resorts, rental shops carry current-season demo gear from real brands, sized and tuned properly, and the queues move faster than people expect once you've booked ahead online. That closes most of the gap that used to make bringing your own the obvious choice. The case for hauling boards or skis across the world is genuinely weaker in Japan than in most ski countries.
Boots are the exception. A rental boot that's close enough is fine for a couple of days, but it's the one piece of gear where a bad fit actually ruins a trip, and Japan's shops size for a narrower average foot than a lot of Western skiers are used to. If you already own boots that fit you properly, bring them and rent everything else.
Where it flips back towards bringing your own: anywhere off the well-worn resort circuit. Rental selection thins out fast once you're away from Niseko, Hakuba or the other big names, and a small local shop might only have a handful of boards in a narrow size range. If your trip is built around quieter, smaller ski areas, check what a specific resort's rental shop actually stocks before assuming it'll sort you out.
The other factor is what it costs to fly gear over versus what a week of rental runs. Airlines vary a lot on ski and board bag fees, and it's worth checking your specific carrier's policy rather than assuming, since some treat it as standard checked luggage and some charge a separate oversized-item fee on top. Weigh that against your resort's actual daily or weekly rental rate before deciding either way.
Outerwear is the easiest call: rent it or bring it, it barely matters. Jackets and pants are simple to size and every shop has plenty, so there's no real fit risk the way there is with boots.