Hi rotarymililani,
Here is the web site of Yamato no Yu (大和の湯) in Narita (成田) :
www.yamatonoyu.com/eng/index.asp (in English).
You need to take the subway to Shimouza Manzaki station (JR Narita, 5 minutes from Narita station) followed by a (short ?) walk. There is a map (also in English) on their web site.
This place is kind of a super-sento, rather that a traditional onsen. The outside bath seems small, as seen on this page :
www.joy.hi-ho.ne.jp/ma0011/T-Chiba05.htm
Note also that while this is a real onsen (because of the water composition), the water is gushing at 18 degrees Celsius, and has to be warmed. But well, it is close to Narita, so probably enjoyable...
sanji