Postcodes of Japan and the data format.
JIS X 0401:1973 To-do-fu-ken (prefecture) identification code JIS X 0402:2003 Identification code for cities, towns and villages
This page describes the odd format of the postcode file and points out where online postcode searches fail to correctly recognize various things in the file.
Blog posts on making a postal code lookup system by the creator of the zipnavi lookup site.
A postcode lookup system.
Using the CSV data from the Japanese post office with the MS access database.