A Positron Emission Mammograph (PEM) with a pair of planer detectors dedicated for breast cancer detection is developed under collaborations of Tohoku University, Kobe City Collage of Technology, and Furukawa Co., Ltd. It uses thin crystals made of a scintillator Pr:Lu3Al5O12 (Pr:LuAG) of 2.1×2.1×15.0 mm in size combined with H8500 position sensitive Photo multi tubes (PMTs) from Hamamatsu. The PEM system is comprised of two opposing detectors that have 140×200 cm2 field of view (FOV). We present its basic architecture with fundamental performance, such as spatial resolution, time resolution, energy resolution, and data acquisition count rate. The Spatial resolution measured using a 22Na point sauce was found to be 1.1 mm FWHM for image planes parallel to the detector faces. Time resolution was 3.1 ns FWHM. And, energy resolution was 11.6 % FWHM (@511 keV) at the center of detector blocks. Coincident count rate was improved dramatically for optimizing energy windows level and implementation a new offset correction method.