First make sure the cap is screwed on tight. If it is, then remove it and check if the o-ring around the cap is in place and isn't twisted. These usually come with a replacement oil filter. It wouldn't hurt to just get a new filter and replace it and the o-rings and torque the cap correctly. If that doesn't work you may need to replace the cap or the housing.
You can see the o-ring in the filter kit:
http://www.autohausaz.com/search/pn....08UaAm6P8P8HAQ