Abstract: [Problem] In quickly assaying a blood component interfered by glucose and/or its derivative on the bedside or in a clinic or in assaying the same by a patient in his/her own home, there has been required an assay method wherein the whole blood can be used as a sample as such without resorting to a centrifuge or the like. [Means For Solving Problems] A method of assaying a blood component to be used for assaying a blood component interfered by glucose and/or its derivative, characterized by comprising bringing the whole blood into contact with a substance capable of converting glucose and/or its derivative into another substance not interfering the assay and, simultaneously or subsequently, separating blood cells; a device to be used in the assay method; and a kit containing this device.