A simulator is provided for simulating a real-water-droplet for an arbitrary simulation time in an entire space to be observed in which the real-water-droplets collide by a certain probability within a predetermined volume in a predetermined time interval, and in which the real-water-droplet is specified by a group of an arbitrary number of attributes and position coordinates at an initial time in one of divided spaces into which the entire space is divided, a super-water-droplet representing a group of an arbitrary number of the real-water-droplets which have a predetermined identical group of the attributes, when the multiplicity which is the arbitrary number of the real-water-droplets represented by the super-water-droplet changes when the super-water-droplet collides with another by a probability based on the certain probability and the multiplicity, calculating data about the super-water-droplet to output data about the real-water-droplet after the arbitrary simulation time elapses.