You can obtain English machine-translations for Japanese patent and utility model documents from 1993 onwards on the Japan Patent Office's free-of-charge "IPDL" (Industrial Property Digital Library) internet service at http://www.ipdl.inpit.go.jp/homepg_e.ipdl.
From the IPDL homepage, choose the "Patent & Utility Model Gazette DB" link. A simple number search will - as a first search result – return the PAJ English abstract, where available. Clicking the "Detail" button on the top of the result screen will start the machine-translation of the original Japanese unexamined application. Alternatively, clicking the "Japanese" button will display the original Japanese document. In cases where no PAJ English abstracts exist you will immediately get the machine-translation from the Japanese.
See the screenshots at http://www.jpo.go.jp/torikumi_e/hiroba_e/paj_15.htm
You can also try Yahoo! Babel Fish - Text Translation and Web Page Translation
LexisNexis offers a fee-based English machine translation service of Japanese patent documents "in minutes" at $39 per translated document (regardless of length).