1. An entertainment apparatus that decides weather of a virtual world displayed on a screen of a display unit, comprising: a global weather deciding means for deciding weather at predetermined time intervals in each global area obtained by dividing a map expressing whole of said virtual world into a plurality of global areas, said weather being decided in accordance with a behavior model that sets a condition for the global area in question and in consideration of area information given in advance to said global area, with said condition being decided from a cyclically-changing event; a local weather deciding means for deciding weather at predetermined time intervals in each local area obtained by dividing each global area into a plurality of local areas, said weather being decided in accordance with weather of the global area in question, and in consideration of area information given in advance to the local area in question, with said weather of the global area being decided by said global weather deciding means; and a weather reflecting means for reflecting weather of a local area on an image of the virtual world displayed on the screen of said display unit, with said image containing said local area, and with said weather being decided by said local weather deciding means.