Using a counter of the Web server 10, a leave probability p1, average value m and variance s2 of think time, and hit ratio r are calculated for a session data cache 12 involving a predetermined Web application. For a first reading of a group of reading plural session data proximate temporally, p1a, ma and s2a, and average value a of the number of data reading sessions in each group are defined. A computational expression setting means 21 sets a computational expression f(a)=a including p1, m, s2, r, p1a, ma and s2a, the computational expression for a fix point computing method having a variable a. A true value searching means 22 searches an almost true value of a by the fix point computing method based on the computational expression f(a)=a. An estimation means 23 estimates ra based on a searched value of a.