To provide an interactive machining program preparing device capable of automatically preparing a machining program for carrying out appropriate circular arc cutting using a rotary cutting tool.

In helical thread cutting performed using a thread cutting tool, the optimum rotating speed of the thread cutting tool T is computed based on optimum cutting speed inputted by an operator and a tool diameter extracted from a tool database, and then cutting feed speed F is computed based on the optimum rotating speed and the tip number and tip feed quantity of the thread cutting tool T. The machining program for correcting the cutting feed speed F to the cutting feed speed F' of the center of the thread cutting tool T is automatically prepared from a distance L between the circular arc center Ow of the cut surface of a workpiece W and the center Ot of the thread cutting tool T, and the circular arc radius R of the cut surface of the workpiece W so that the cutting edge of the thread cutting tool T helically moves at the cutting feed speed F.