  In this paper, we describe a newly discovered statistical property of time
series data for daily price changes. We conducted quantitative investigation of
the {\it calm-time intervals} of price changes for 800 companies listed in the
Tokyo Stock Exchange, and for the Nikkei 225 index over a 27-year period from
January 4, 1975 to December 28, 2001. A calm-time interval is defined as the
interval between two successive price changes above a fixed threshold. We found
that the calm-time interval distribution of price changes obeys a power law
decay. Furthermore, we show that the power-law exponent decreases monotonically
with respect to the threshold.
  Keyword: econophysics, stock price changes, calm time interval, power-laws;
PACS: 89.90.+n; 05.40.Df;
