From 1901 to 1926, the League of Silk Industry (Seishi-domei) in Suwa district had the institution of registering women labourers who worked in the industry there. Most students of the League have regarded this institution as perfectly functioning. According to them, labourers who were employed at one of the factories of the League could not move to another. But such a view is not correct. The fact was that not a few labourers neglected this institution and that the manufacturers could not or would not observe the rule strictly. This paper will clarify on the evidence to what extent this institution worked, and will suggest the problem of the making of labourers' spontaneous norm in Japan that one should not change one's employer.