This vintage 1960s shopping mall earned Nakano its reputation as an underground Akihabara: it's home to the original Mandarake Complex, the famous manga and anime goods shop, plus lots more stores that sell collectables of all sorts (from zines and vintage toys to antique watches and army surplus).
At the south entrance look for the small kiosk that has the regularly updated Nakano Broadway Guidebook, which lists all the shops.