Hi Satoaki,
Matt has given you the first important step to get quality recordings from your organ. All organs that Technics has made from the FA1 on have the same operating system. And, all have a very low output from the organ. As a result most organists producing high quality recording utilize some type of pre amp going into the recorder. Just a simple mixer will give an adequate boost to the signal to do the job. I recommend saving your recorded song to a disc so that levels can be adjusted to accommodate the dynamic limitations of the recorder.
Frank