A sample analyzer comprising: a sample preparing means configured to prepare a measurement sample by mixing a sample and reagent; a measuring means configured to irradiate light on the measurement sample and obtain time series data representing the change over time of the optical data values; a controller; wherein the controller is configured to determine the start point and the end point of a linear interval of the obtained optical data values, wherein the start point is determined by setting a start point detection interval shorter than the acquisition period of the time series data obtained by the measuring means at the beginning of the acquisition period of the time series data; determining a data value as the start point of the start point detection interval during which the time series data vary linearly when the data values contained in the set start point detection interval satisfy a first evaluation criteria related to the distribution of the data values by sequentially shifting the start point detection interval one period forward until the data values contained in the set start point detection interval satisfy the first evaluation criteria; wherein the end point is determined after the start point has been determined to be a data value after the start point and which satisfies a second evaluation criteria related to linearity of said linear interval; and said controller is further configured to determine the concentration of a target material contained in the sample based on a value representing the slope of a regression line which is based on the data values included in the linear interval from the start point to the endpoint, wherein the first evaluation criteria uses, as an index, the proportion of the data values which are lower or higher values than the base line connecting the initial data value and the final data value of the start point detection interval among all data values within the start point detection interval; the controller is configured to determine whether the first evaluation criteria is satisfied based on the comparison of the proportion and a first threshold value. a sample preparing means configured to prepare a measurement sample by mixing a sample and reagent; a measuring means configured to irradiate light on the measurement sample and obtain time series data representing the change over time of the optical data values; a controller; wherein the controller is configured to determine the start point and the end point of a linear interval of the obtained optical data values, wherein the start point is determined by setting a start point detection interval shorter than the acquisition period of the time series data obtained by the measuring means at the beginning of the acquisition period of the time series data; determining a data value as the start point of the start point detection interval during which the time series data vary linearly when the data values contained in the set start point detection interval satisfy a first evaluation criteria related to the distribution of the data values by sequentially shifting the start point detection interval one period forward until the data values contained in the set start point detection interval satisfy the first evaluation criteria; wherein the end point is determined after the start point has been determined to be a data value after the start point and which satisfies a second evaluation criteria related to linearity of said linear interval; and said controller is further configured to determine the concentration of a target material contained in the sample based on a value representing the slope of a regression line which is based on the data values included in the linear interval from the start point to the endpoint, wherein the first evaluation criteria uses, as an index, the proportion of the data values which are lower or higher values than the base line connecting the initial data value and the final data value of the start point detection interval among all data values within the start point detection interval; the controller is configured to determine whether the first evaluation criteria is satisfied based on the comparison of the proportion and a first threshold value.