PROBLEM TO BE SOLVED: To provide an automatic irrigation system which enables irrigation timing to be determined without measuring a moisture content in the soil in the field and quantity of solar radiation to the field.SOLUTION: An automatic irrigation system comprises a water supply device 10 and a control device 30. The control device 30 includes a processing unit 31 and an input unit 32. To the input unit 32, information on a maximum irrigation amount, that is a maximum amount of water a field 21 can retain, and information on a disappearance amount, that is a disappeared amount of water from the field 21 after irrigation to the field 21, are input. The processing part 31 generates instruction content that first irrigation is performed at start of prescribed period after seeding of plant 20 to the field 21, and second and subsequent irrigation are performed from the next day or later of the day first irrigation is performed, until end of the prescribed period. The processing unit 31 determines timings of second and subsequent irrigation, by using: value obtained by subtracting a first supply amount that is an amount of supplied water in the first irrigation, from the maximum irrigation amount and the disappearance amount.
