To provide a device for calculating the traveling of the interface in three-dimensional fusion or solidification by using a computer, the execution time of which can be made shorter than that of the conventional calculation method and to provide a method for moving the interface.

This device for calculating the traveling of the interface in three-dimensional fusion or solidification by using the computer is provided with a means (a) for inputting the temperatures, coordinate values and physical property values of four positions near the interface, a means (b) for calculating the temperature gradient value near the inputted interface, a means (c) for calculating the traveling distance of the interface by using the temperature gradient value calculated by the means (b), a means (d) for calculating the processing for traveling the temperature gradient value calculated by the means (b) to a suitable direction, a calculating means (e) for changing a discontinuous point to a continuous point and a means (f) for displaying the position of the traveled interface as an image.