Craig Brown, Scotland's manager, is being forced to take something of a shadow squad because of the Tennents Scottish Cup final, Scottish and English League play-offs and injuries.
However, Miller, standing in at yesterday's squad announcement for the absent Brown, who is at a coaching course in Europe, said it was important preparation for the European Championship qualifier in the Faroe Islands on 7 June.
Miller argued that there are six regulars in a 13-man party. Five more players will be added after it is decided on Saturday which sides will be involved in Scotland's first promotion-relegation play-off, which takes place at the same time as the tour.
The uncapped England-based trio, Paul Bernard of Oldham, Craig Burley of Chelsea, and Scot Gemmill of Nottingham Forest were named in the initial 13.
"We've got six regulars there and it's important that we keep fitness levels right before the Faroes game in June." Miller said. "For players who've been in a B squad or stepped up from the under-21s, it's a chance to impress.
"And it's also good experience of a tournament nature because we've several players who haven't been involved in that with Scotland before."
The Blackburn defender Colin Hendry looks favourite to captain Scotland in the absence of Gary McAllister of Leeds, who is to have an operation, and Paul McStay, who will be involved in the cup final for Celtic.
Scotland face Japan in Hiroshima on Sunday 21 May then meet Ecuador in Toyama on 24 May. The full squad of 18 players will leave from Glasgow next Wednesday 17 May.
The five players to be added to the squad after League matches this weekend will be chosen from: Crawford (Raith), Lambert (Motherwell), Levein (Hearts), McKimmie (Aberdeen), McKinlay (Dundee Utd), Robertson (Hearts), Shearer (Aberdeen).
SCOTLAND SQUAD (for the Kirin Cup in Japan, May 21-28): Leighton (Hibernian), Woods (Motherwell); Bernard (Oldham), Burley (Chelsea), Calderwood (Tottenham), Gemmill (Nottm Forest), Hendry (Blackburn), Jackson (Hibernian), McKinnon (Motherwell), McLaren (Rangers), Martin (Motherwell), Spencer (Chelsea), Walker (Partick).