An automatic analyzer comprising: a reagent storage including plural slots capable of storing reagent containers each having a main body housing a reagent used for analysis, an opening provided on a top of the main body and a lid portion having a sealing member for sealing the opening by being inserted into the opening; a reagent loader mechanism for setting in the reagent containers from the outside of the automatic analyzer and being capable of carrying the reagent container in the reagent storage; a processing mechanism executing dispensing or stirring processing to the reagents stored in the reagent storage; and a reagent container lid opening/closing mechanism provided in the reagent storage, opening and closing lids of the reagent containers set in the reagent storage before and after the processing mechanism performs the processing; characterized by further comprising: a reagent container lid half-open mechanism configured to achieve a half-open state of the opening of the reagent container when performing an operation of setting the reagent container in the reagent loader mechanism from the outside of the automatic analyzer by: - removing the sealing member from the opening of the reagent container, and - opening the lid portions at a predetermined angle, said predetermined angle being such that the lid portion still covers the opening; wherein the reagent container lid half-open mechanism is provided in the middle of a path through which the reagent container passes when performing the operation of setting the reagent container in the reagent loader mechanism from the outside of the automatic analyzer. a reagent storage including plural slots capable of storing reagent containers each having a main body housing a reagent used for analysis, an opening provided on a top of the main body and a lid portion having a sealing member for sealing the opening by being inserted into the opening; a reagent loader mechanism for setting in the reagent containers from the outside of the automatic analyzer and being capable of carrying the reagent container in the reagent storage; a processing mechanism executing dispensing or stirring processing to the reagents stored in the reagent storage; and a reagent container lid opening/closing mechanism provided in the reagent storage, opening and closing lids of the reagent containers set in the reagent storage before and after the processing mechanism performs the processing; characterized by further comprising: a reagent container lid half-open mechanism configured to achieve a half-open state of the opening of the reagent container when performing an operation of setting the reagent container in the reagent loader mechanism from the outside of the automatic analyzer by: - removing the sealing member from the opening of the reagent container, and - opening the lid portions at a predetermined angle, said predetermined angle being such that the lid portion still covers the opening; wherein the reagent container lid half-open mechanism is provided in the middle of a path through which the reagent container passes when performing the operation of setting the reagent container in the reagent loader mechanism from the outside of the automatic analyzer. a reagent container lid half-open mechanism configured to achieve a half-open state of the opening of the reagent container when performing an operation of setting the reagent container in the reagent loader mechanism from the outside of the automatic analyzer by: - removing the sealing member from the opening of the reagent container, and - opening the lid portions at a predetermined angle, said predetermined angle being such that the lid portion still covers the opening; - removing the sealing member from the opening of the reagent container, and - opening the lid portions at a predetermined angle, said predetermined angle being such that the lid portion still covers the opening; wherein the reagent container lid half-open mechanism is provided in the middle of a path through which the reagent container passes when performing the operation of setting the reagent container in the reagent loader mechanism from the outside of the automatic analyzer.