Provided are a measurement sample diluent that enables the measurement of the proportion of HbA1c in the total Hb molecules in a measurement sample (HbA1c (%)), with high sensitivity, high accuracy, and high correlation with HPLC without being affected by the length of time required to mix a measurement sample with the measurement sample diluent and to drop the mixture onto an immunochromatographic specimen for measuring HbA1c; and a kit containing the measurement sample diluent. The present invention relates to a measurement sample diluent for immunochromatography for quantifying the proportion of hemoglobin A1c in the total hemoglobin molecules in a measurement sample (hemoglobin A1c (%)); and the measurement sample diluent is an aqueous solution that contains a non-ionic surfactant, an anionic surfactant, and a buffer.