An automatic analysis device for analyzing the blood clotting time of a sample, comprising: a sample vessel placing section in which a sample vessel containing the sample to be analyzed is to be placed; a reagent vessel placing section in which a reagent vessel containing a reagent to be used for measuring the sample is to be placed; a reaction vessel in which the reagent and the sample are to be mixed and reacted; a sample dispensing mechanism adapted to dispense the sample into the reaction vessel from the sample vessel; a reagent dispensing mechanism adapted to dispense the reagent into the reaction vessel from the reagent vessel; a detector adapted to detect a signal value that changes over time in accordance with the degree of the mixing reaction of the sample and the reagent; and a control section adapted to analyze the sample based on the result of the detection, wherein the signal value is any one of the amount of transmitted light, the amount of scattered light, the amount of fluorescence, and the turbidity; and the control section includes: a signal reference value setting control section adapted to create a blood clotting reaction curve obtained from the signal value changing over time in a mixing reaction of a blood clotting time reference sample having a known expected blood clotting time and the reagent, and to set the value of the blood clotting reaction curve at the known expected blood clotting time as a signal reference value; a storage section adapted to store the set signal reference value together with a lot number of the reagent as identification information for the reagent, and a processing section adapted to determine the blood clotting time of the sample as the time when the signal value detected by the detector exceeds the stored signal reference value, characterized in that the signal reference value setting section is adapted to set a plurality of signal reference values based on the reaction curve of the blood clotting time reference sample at a plurality of known concentrations, to determine an approximate formula from a relationship between the set plurality of signal reference values and the blood clotting time, and to set a signal reference value function which varies depending on the concentration of the blood clotting time reference sample based on said approximate formula. a sample vessel placing section in which a sample vessel containing the sample to be analyzed is to be placed; a reagent vessel placing section in which a reagent vessel containing a reagent to be used for measuring the sample is to be placed; a reaction vessel in which the reagent and the sample are to be mixed and reacted; a sample dispensing mechanism adapted to dispense the sample into the reaction vessel from the sample vessel; a reagent dispensing mechanism adapted to dispense the reagent into the reaction vessel from the reagent vessel; a detector adapted to detect a signal value that changes over time in accordance with the degree of the mixing reaction of the sample and the reagent; and a control section adapted to analyze the sample based on the result of the detection, wherein the signal value is any one of the amount of transmitted light, the amount of scattered light, the amount of fluorescence, and the turbidity; and the control section includes: a signal reference value setting control section adapted to create a blood clotting reaction curve obtained from the signal value changing over time in a mixing reaction of a blood clotting time reference sample having a known expected blood clotting time and the reagent, and to set the value of the blood clotting reaction curve at the known expected blood clotting time as a signal reference value; a storage section adapted to store the set signal reference value together with a lot number of the reagent as identification information for the reagent, and a processing section adapted to determine the blood clotting time of the sample as the time when the signal value detected by the detector exceeds the stored signal reference value, a signal reference value setting control section adapted to create a blood clotting reaction curve obtained from the signal value changing over time in a mixing reaction of a blood clotting time reference sample having a known expected blood clotting time and the reagent, and to set the value of the blood clotting reaction curve at the known expected blood clotting time as a signal reference value; a storage section adapted to store the set signal reference value together with a lot number of the reagent as identification information for the reagent, and a processing section adapted to determine the blood clotting time of the sample as the time when the signal value detected by the detector exceeds the stored signal reference value, characterized in that the signal reference value setting section is adapted to set a plurality of signal reference values based on the reaction curve of the blood clotting time reference sample at a plurality of known concentrations, to determine an approximate formula from a relationship between the set plurality of signal reference values and the blood clotting time, and to set a signal reference value function which varies depending on the concentration of the blood clotting time reference sample based on said approximate formula.