Diapet is the name of a series of mostly 1:40 scale die-cast toys produced by Yonezawa toys from Japan after that company took over the Cherryca Phenix brand. While Yonezawa is best known for producing tinplate toys of particular accuracy and in the scale of 1:24-1:18, Diapet toys are smaller die-cast replicas. Their quality and realism made them popular for Asian children and collectors alike, although they have never made much of an impact in America. The word "pet" is the Japanese equivalent of "small" or "cute." Diapet toys continued being produced through the 1990s.
Other popular die-cast brands to come out of Japan include Asahi Model Pet, Miniature Pet, Micropet, Cherryca Phenix, Tomica and Collectoy. Of them, Tomica (from Tomy of Japan) is the best-known to US collectors.
What's it worth? Take a look at this Diapet price guide: sold listings for a value indication.