[edit: shanxi posted whilst I was typing]
It is a 'billon tetradrachm' issued by
Diocletian in
Alexandria in
Egypt.
There are several books which can be used to identify these, such as Milne's
Catalogue of Alexandrian Coins (old, originally published around 1933 I think, but reprinted several times), but most are now also listed in the new volumes of Sear's
Roman Coins and their Values (volume 4 in this case).
The main difficulty in identifying them is that the legends are in Greek, so yours reads ΔIOKΛHTIANOC CЄB for 'Diocletian Augustus'. If you post the
reverse someone will identify it more precisely for you: they are normally dated to each year of the reign, but there are several different
reverse types each year.
Best wishes
Alan
