Scotland turned in a dominant second-half display in Gloucester, running in five tries to secure the bonus-point to leave them well placed in Pool B after just one game.
The margin of victory suggests a comfortable afternoon for the Scots but it was only after the break where they exerted their superiority over a side still revelling in their stunning success over the Springboks.
The Scots raced into an early 6-0 lead but Japan did hit back with the first try through Amanaki Mafi before further penalties from Greg Laidlaw saw Vern Cotter's men go in at half-time 12-7 ahead.
Then, two tries from Mark Bennett, and further efforts from Finn Russell, Tommy Seymour and John Hardie sealed it for the Scots overall.
Laidlaw says the victory was a great way for Scotland to begin the tournament, describing the battling display as "determined and outstanding".
He said: "Happy days with that result - but that is just the start hopefully for this team. We talked at length about playing for 80 minutes and we have done that today.
"I thought we were outstanding for the duration. They said if they were in game at half-time they could take us, so we were determined and outstanding."
He added: "Credit to Japan though, they were awesome against South Africa and really tough opponents again today. That win against South Africa blew the group wide open and meant it was important for us to win today."
Cotter's men play the USA next on Sunday.