To achieve improvement in injection quantity adjustment accuracy and reduction in adjustment time in a method for adjusting the injection quantity of an injector.

A method for adjusting the injection quantity of an injector is configured such that the pulse width and the injection quantity are multipoint measured for each adjustment target injector in order to calculate the relationship between the pulse width and the injection quantity and to calculate an inclination α as an adjustment factor for each adjustment target injector (S1, S2). That is, it is configured to understand the required pulse width Ta, required for obtaining the reference injection quantity T0, by using the inclination α calculated from injection characteristics measured in the adjustment target injector. Therefore, compared with the case of using an inclination α_j calculated from the reference injection characteristics like a conventional one, it is possible to further accurately understand the required pulse width Ta. The injection quantity measured with the required pulse width Ta does not vary within the standard.