Image-projective Desktop Arm Trainer IDAT is designed to improve the upper limb therapy effect. This paper is focused on its design concept. The most important structural feature of IDAT is that it is non-invasive and unconstrained. Setting on the desk apart from the trainee sitting makes no body contact. It works based on the trainee's voluntary movement, although it has neither physical assist nor tactile feedback on conventional systems. IDAT is developed based on the design concept that the direct interaction is critical in terms of the motivation increase. Instead of a joystick, a handle, a robotic arm and a display screen on conventional systems, IDAT uses the screen projected on a desktop and produces the visual reaction at the time on the spot where the trainee operates. It inspires the vivid, lively and actual feeling to the trainee.