I have been thankful to those who kindly answered to my previous question.Despite of your advice, I had been lazy to skip reply. Allow me to thank you very much for the help that you rendered.
After that, I was able to send and recive data via Bluetooth SPP in reference to the indicated information on your message.（on Debian 210 SnapShot/233）
But,I still have another problem with me.
What I did was to connect a PC(Windows) with DragonBoard by a USB cable to conduct serial communication in between.
Thereafter, I used an Android smartphone which was connected with the DragonBoard by Bluetooth to input the following command to PC Terminal(TeraTerm);
echo "******" > /dev/rfromm0
Then I confirmed that the character string data was transferred, as I noticed the same text appeared on the Android app.
On top, I was also able to receive data sent from Android app on PC Terminal(TeraTerm) via rfcomm0, with using a serial data receiving program on the PC terminal.
However, if I repeat the same procedure on a standard terminal of DragonBoard (XTerm was used), the following message appears;
/dev/rfcomm0: Device or resorce busy
and I cannot send and recieve the data.
What I guess is that this may be a different problem than Bluetooth SPP and I am afraid that it may be irrelevant to post a question to you here.
But if there is anyone who has some idea of possible solution on this case, could you be so kind to share it with me?