I saw this plant at the edge of some woods during our walk in a satoyama valley, and then again in someone’s garden. This one is called Polygonatum odoratum ソロモンズシール (see this page). It is quite similar to the Smooth Solomon’s Seal which is from America Polygonatum biflorum. The plant is related to the convallaria Lily of the Valley.
Why the plant is called Solomon’s Seal: In the winter when the leaf stalk dies back and becomes completely removed from the rhizome, a scar remains which is said to resemble the Seal of King Solomon, who reined as King of Israel from 961 – 931 BC (see this page for more on its history). The seal is a hexagram composed of two overlapping triangles to create a six pointed star, commonly called the Star of David after King David, father of Solomon. The seal became synonymous with Solomon who apparently used the symbol to cast away demons and summon angels.
References: Solomon’s Seal; Botanical.com