purely Japanese style
Used to describe something that is entirely in a traditional Japanese manner, without Western or foreign influence.
This inn is built in a purely Japanese style, with tatami rooms and shoji screens.
和風 means 'Japanese-style' broadly, while 純日本式 emphasizes pure, unadulterated Japanese style without foreign elements.
Compound of 純 (pure) + 日本 (Japan) + 式 (style). The exact historical derivation is uncertain, but the word transparently means 'pure Japanese style'.