An automated analyzer for mixing a specimen and a reagent and measuring a change in a mixture of the specimen and the reagent with time, comprising: a measurement point data acquisition unit which acquires a plurality of measurement point data (130, 140, 150) from a reaction curve of the specimen and the reagent; a data processing unit which processes the measurement point data; a storage unit which stores a first approximation formula used by the data processing unit; and an output unit which outputs a processing result from the data processing unit, wherein the first approximation formula is one of the following Expressions 1 to 4: x = a * t + b + c * exp − k * t x = a * t + b + e / t + d x = a * t + b + w / exp u*t + v x = a * t + b + p * log 1 + q * exp r * t where t represents a time, x represents absorbance, and a, b, c, d, e, k, p, q, r, u, v, and w are parameters, characterized in that the data processing unit causes the first approximation formula stored in the storage unit to approximate the plurality of measurement point data (130, 140, 150) in order to arrive at a second approximation formula that is obtained by optimizing the values of the parameters, calculates a straight line, which is approached by said second approximation formula, and determines a time, at which a difference between the straight line and the second approximation formula falls below a reference value (ε) or at which a difference between a slope of the straight line and of the second approximation formula falls below a reference value (δ), as a start time of the linear range of the reaction curve. a measurement point data acquisition unit which acquires a plurality of measurement point data (130, 140, 150) from a reaction curve of the specimen and the reagent; a data processing unit which processes the measurement point data; a storage unit which stores a first approximation formula used by the data processing unit; and an output unit which outputs a processing result from the data processing unit, wherein the first approximation formula is one of the following Expressions 1 to 4: x = a * t + b + c * exp − k * t x = a * t + b + e / t + d x = a * t + b + w / exp u*t + v x = a * t + b + p * log 1 + q * exp r * t where t represents a time, x represents absorbance, and a, b, c, d, e, k, p, q, r, u, v, and w are parameters, characterized in that the data processing unit causes the first approximation formula stored in the storage unit to approximate the plurality of measurement point data (130, 140, 150) in order to arrive at a second approximation formula that is obtained by optimizing the values of the parameters, calculates a straight line, which is approached by said second approximation formula, and determines a time, at which a difference between the straight line and the second approximation formula falls below a reference value (ε) or at which a difference between a slope of the straight line and of the second approximation formula falls below a reference value (δ), as a start time of the linear range of the reaction curve. where t represents a time, x represents absorbance, and a, b, c, d, e, k, p, q, r, u, v, and w are parameters, characterized in that the data processing unit causes the first approximation formula stored in the storage unit to approximate the plurality of measurement point data (130, 140, 150) in order to arrive at a second approximation formula that is obtained by optimizing the values of the parameters, calculates a straight line, which is approached by said second approximation formula, and determines a time, at which a difference between the straight line and the second approximation formula falls below a reference value (ε) or at which a difference between a slope of the straight line and of the second approximation formula falls below a reference value (δ), as a start time of the linear range of the reaction curve.