There is one famous "ONSEN" Inn place available in Chiba prefecture, which is called "YORO-KEIKOKU" in the middle of Chiba. You may take JR Keiyo Line from Tokyo station to Goi station in Ichihara city, and switch to Kominato Line to reach Yoro-Keikoku station. It takes a little longer time in 2.5 hours to reach there, however, it is worthwhile for you to consider your visit.
For those residents in Tokyo, they normally look for ONSEN Inns near Hakone, Kanagawa prefecture, when they like to go wihtin a limited time. Comparatively, Chiba was not well known to people as a good ONSEN Inn place.
In particular for those who like to take a day-trip to enjoy ONSEN, Chiba used not to be selected by tourists. However, it has been changed 10 years ago, since a newly-opened "GORIYAKU-NO-YU" (so-called "Fortuned ONSEN Inn") started to serve those "Day-Trip" tourists well.
If you happen to stay at Tokyo for either business or private trip and you spend one weekend, you may consider to go and visit "YORO-KEIKOKU" for a day-trip to relax yourself at ONSEN. The entry fee at GORIYAK-NO-YU" is Yen1,200 (US$10) only. If you have a chance in Autum season, you could enjoy "ROTEN-BURO" (external ONSEN bath) by looking at "KOYO" (Autumn Leaves) in the end of Nov to mid of Dec every year. "GORIYAKU-NO-YU" web site is http://www.goriyakunoyu.jp/.