The incidence of breast cancer is in the first place as a cause of death among Japanese women. The breast cancer is a type of cancer which can be cured relatively easily when detected early. However, it is now clear that the detection of breast cancer is difficult without imaging modalities such as X-ray mammography and/or ultrasonic echography. These imaging devices are useful, however, morphological information only are not enough for differentiating cancer from other benign diseases. Positron emission tomography (PET) for whole-body diagnosis can give us functional information regarding the degree of malignancy. But still there remain various problems such as limited spatial resolution and motion artifacts due to respiratory movement, as well as an issue of the high costs. To compensate these weak points, we have started development of the positron emission mammography (PEM) dedicated for the local diagnosis of the breast tissue. We had developed a test device, "PEM simulator" last year. And we have developed a prototype PEM scanner. In the present paper, basic features are introduced regarding a gantry, chair, detector unit and measurement system of the PEM scanner, as well as our first results obtained by using this prototype scanner.