I'd say tech salaries highly depend on your company and even the type of position you're in.
At foreign FAANG (which means Google and Amazon here) and investment banks traditionally you can expect upwards of 10M JPY/y (total compensation) for moderately senior engineering positions and even over 15M for more senior positions. But it's not just foreign companies. Some Japanese companies like Mercari and Fast Retailing are now in an engineer hiring craze and they're offering highly competitive salaries.
My personal history in Japan: 2015-2016 8M JPY 2017 12M JPY (renegotiated my salary) 2018 13M JPY 2019 14M JPY