An automatic analyzer, comprising: a sample nozzle dispensing a sample into a reaction container ; a reagent nozzle dispensing a reagent into the reaction container ; a reaction disk including the reaction container for mixing the sample and the reagent; a pressure changing mechanism (4, 7; 17; 18; 19) changing pressure in either one of the sample nozzle and the reagent nozzle ; a liquid feeding mechanism feeding a cleaning liquid to the nozzle ; a pipe connecting the nozzle , the pressure changing mechanism (4, 7; 17; 18; 19), and the liquid feeding mechanism ; and a controller which controls the pressure changing mechanism (4, 7; 17; 18; 19) and the liquid feeding mechanism , wherein the controller cleans the inside of the nozzle by feeding the cleaning liquid from the liquid feeding mechanism into the nozzle , the controller decelerates a flow velocity of the cleaning liquid fed from the liquid feeding mechanism by controlling the pressure changing mechanism (4, 7; 17; 18; 19) at the time of cleaning the inside of the nozzle , characterized in that the controller accelerates the flow velocity by the pressure changing mechanism (4, 7; 17; 18; 19) immediately after decelerating the flow velocity of the cleaning liquid by the pressure changing mechanism (4, 7; 17; 18; 19), the controller controls the pressure changing mechanism (4, 7; 17; 18; 19) so that the sum of absolute values of fluctuation rates of the average flow velocity during the period of acceleration and deceleration by the pressure changing mechanism (4, 7; 17; 18; 19) becomes 20% or more in comparison with the state where the pressure changing mechanism (4, 7; 17; 18; 19) is not driven, and the controller controls the liquid feeding mechanism to continue feeding cleaning liquid into the nozzle during the process of deceleration and acceleration. a sample nozzle dispensing a sample into a reaction container ; a reagent nozzle dispensing a reagent into the reaction container ; a reaction disk including the reaction container for mixing the sample and the reagent; a pressure changing mechanism (4, 7; 17; 18; 19) changing pressure in either one of the sample nozzle and the reagent nozzle ; a liquid feeding mechanism feeding a cleaning liquid to the nozzle ; a pipe connecting the nozzle , the pressure changing mechanism (4, 7; 17; 18; 19), and the liquid feeding mechanism ; and a controller which controls the pressure changing mechanism (4, 7; 17; 18; 19) and the liquid feeding mechanism , wherein the controller cleans the inside of the nozzle by feeding the cleaning liquid from the liquid feeding mechanism into the nozzle , the controller decelerates a flow velocity of the cleaning liquid fed from the liquid feeding mechanism by controlling the pressure changing mechanism (4, 7; 17; 18; 19) at the time of cleaning the inside of the nozzle , characterized in that the controller accelerates the flow velocity by the pressure changing mechanism (4, 7; 17; 18; 19) immediately after decelerating the flow velocity of the cleaning liquid by the pressure changing mechanism (4, 7; 17; 18; 19), the controller controls the pressure changing mechanism (4, 7; 17; 18; 19) so that the sum of absolute values of fluctuation rates of the average flow velocity during the period of acceleration and deceleration by the pressure changing mechanism (4, 7; 17; 18; 19) becomes 20% or more in comparison with the state where the pressure changing mechanism (4, 7; 17; 18; 19) is not driven, and the controller controls the liquid feeding mechanism to continue feeding cleaning liquid into the nozzle during the process of deceleration and acceleration. <EOS>