A noninvasive blood radioactivity monitor system for Positron Emission Tomography (PET) study has been developed. This system has dual plastic scintillators to detect positrons from the wrist artery. One is for monitoring the blood radioactivity in artery and tissue, and another is for monitoring only in tissue, in order to subtract background radiation from tissue. We carried out phantom experiments for evaluating basic characteristics of this monitor system. Clinical experiments using 15O-labeled water were also done to compare this system with a conventional invasive monitor.