PROBLEM TO BE SOLVED: To provide growth diagnostic system and method for large sized algae capable of diagnosing deterioration of a growth state of large sized algae in an initial stage.SOLUTION: The growth diagnostic system for large sized algae is configured so that analysis means calculates average fluorescence life of fluorescence in 580 nm wavelength (step S2). Then growth diagnostic result output means compares the average fluorescence life of the fluorescence in 580 nm wavelength calculated by the analysis means in the step S2, and a threshold value stored in storage means, and when the average fluorescence life is equal to or more than the threshold value (step S3, YES), the growth diagnostic result output means outputs abnormality (step S4).
