Michinobu is referring to the famous Japanese tale of Urashima Tarō, who married the daughter of the Sea-Dragon King and spent time with her in king’s palace beneath the sea. When he asked to pay a visit home, his wife gave him a jewelled box and strict instructions not to open it while on land. Of course, his curiosity gets the better of him and he does so, only to age to death in an instant. The tale had given rise to a superstition that opening a box belonging to someone else could bring bad luck and prevent friends from meeting again, so Michinobu is saying, ‘I haven’t opened the box, as I want to be able to see you again, my friend.’
You can find a poetic version of the Urashima Tarō tale here.