1. A computer system which handles web requests received from clients, comprising: a request classification log generating tool generating a request classification log which describes a number of web requests handled by said computer system in each of at least one measurement period for each of at least one type of said web requests; a system log generating tool generating a system log which describes a status of resources of said computer system in said each measurement period; and a resource usage status calculation tool, implemented by a hardware processor, calculating a request handling resource usage amount per web request by using said request classification log and said system log for each type of said web requests, the request handling resource usage amount being a usage amount of resources used for handling said web requests, wherein said resource usage status calculation tool calculates said request handling resource usage amount through performing multiple linear regression analysis on a number of web requests of each type of said web requests in said each measurement period described in said request classification log and a usage status of resources of said computer system described in said system log for said each measurement period, wherein a web layer is formed by at least one web server in said computer system, wherein said multiple linear regression analysis are performed for measurement periods “1” to “m”, m being a measurement number of times, wherein a type of each of said web requests is any of web request types “R 1 ” to “Rn”, n being the number of types of said web requests, wherein a CPU time U W Rj of the web request type “Rj” and Δt·ρ z w are calculated for i ranging from 1 to n in accordance with the following equation: [ U R ⁢ ⁢ 1 W U R2 W ⋮ U Rn W Δ ⁢ ⁢ t · ρ Z W ] = ( t ⁢ X X ) - 1 t ⁢ X ⁡ [ Δ ⁢ ⁢ t · ρ W ⁡ ( 1 ) Δ ⁢ ⁢ t · ρ W ⁡ ( 2 ) ⋮ Δ ⁢ ⁢ t · ρ W ⁡ ( m ) ] , where Δt is a length of each of said measurement periods, ρ w (i) is a CPU usage rate for entire said web layer for the measurement period “i”, ρ z w is a CPU usage rate for the entire said web layer in a situation in which no web request is handled, and t X is a transposed matrix of matrix X represented by the following equation: X = [ N R ⁢ ⁢ 1 ⁡ ( 1 ) N R ⁢ ⁢ 2 ⁡ ( 1 ) … N Rn ⁡ ( 1 ) 1 N R ⁢ ⁢ 1 ⁡ ( 2 ) N R ⁢ ⁢ 2 ⁡ ( 2 ) … N Rn ⁡ ( 2 ) 1 ⋮ ⋮ ⋱ ⋮ ⋮ N R ⁢ ⁢ 1 ⁡ ( m ) N R ⁢ ⁢ 2 ⁡ ( m ) … N Rn ⁡ ( m ) 1 ] , N Rj (i) being a number of treated web requests of the web request type “Rj” in the measurement period “i”, wherein a memory usage amount M rj w of the web request type “Rj” and a memory use amount M Z W in said web layer in the situation in which no web request is handled are calculated for i ranging from 1 to n by using the following equation: [ M R ⁢ ⁢ 1 W M R2 W ⋮ M Rn W M Z W ] = ( t ⁢ X X ) - 1 t ⁢ X ⁡ [ M W ⁡ ( 1 ) M W ⁡ ( 2 ) ⋮ M W ⁡ ( m ) ] , wherein M W (i) is a memory usage amount for the entire said web layer for the measurement period “i”, and wherein m is set to a value greater than the number n of the types of the web requests so that an inverse matrix of the matrix ( t X X) exists.