PURPOSE:To evaluate the grain mode maturity in the state of thread by measuring a mottle signal of thread for variable density mottle of dyeing for a certain section, and calculating the frequency distribution of data of a deep dyeing corresponding part in accordance with the mottle signal, and comparing it with a reference value. CONSTITUTION:Thread which is subjected to heat treatment slightly and is crystallized partially in the spinning process is drawn at a draw ratio higher than the normal draw ratio in the drawing process and is allowed to pass an evenness tester 1. The output of this tester 1 is sent to an A/D converter 3 through a BPF 2, and the digital output is supplied to a personal computer 4. An FF 5 is connected to the computer 4 to extract measured voltage data at intervals of a prescribed time. This measured voltage data is compared with a reference voltage to calculate a deep dyeing duration of the deep dyeing corresponding part by the computer 4, and the frequency distribution of this value is generated to perform printing by a printer 6. Thus, the grain mode maturity is estimated and evaluated.