Mariah Carey -- MAJOR Voice Problems During Concert in Japan

Mariah Carey struggled to get through the first night of her tour Saturday evening in Japan ... with her voice giving out -- and the video is PAINFUL to watch.

Carey kicked off her Elusive Chanteuse World Tour in Tokyo ... and based on the video -- as well as multiple others online -- it's clear she couldn't hit the high notes she's famous for.

During some portions of the concert, it seemed like Mariah forgot the lyrics ... however it's possible her microphone gave out -- since the stand broke during her performance of "We Belong Together."

Heartbreaker.