Provided is a method for measuring a blood component amount that can sufficiently and properly correct the blood component amount by measuring an Hct value with high accuracy and high reliability, and a sensor and a measuring device that are used in the method.A method for measuring a blood component amount uses a biosensor to calculate a blood component amount in blood. The biosensor includes the following: a first electrode system having a first working electrode and a first counter electrode; a second electrode system having a second working electrode and a second counter electrode; and a reagent portion arranged in a form that covers at least a part of the first electrode system, but does not cover the second working electrode. The method includes the following: a first step of detecting a first current value that flows through the first electrode system and calculating an apparent blood component amount in the blood based on the first current value, during a period in which a first voltage is applied to the first electrode system and a second voltage is applied to the second electrode system while the first voltage is being applied; followed by a second step of stopping the application of the first voltage to the first electrode system, applying a third voltage to the second electrode system, and detecting a second current value; and a step of calculating a true blood component amount using the apparent blood component amount and the second current value.