[en] MU is the abbreviation of monitor unit, which is the count number of a dosimeter that monitors the beams at the irradiation beam emitting part of an external radiation therapy device. The system is such that irradiation is stopped when the monitor dosimeter reaches the setting MU. This paper explained the MU verification method for high-energy X-rays and the construction of a plan verification system including MU verification. First, it explained the reason why MU verification is necessary. As MU verification methods, there are (1) actual measurement using a phantom, (2) manual calculation using a basic data table, (3) use of general-purpose spreadsheet software, (4) exclusive use of MU verification software, and (5) other treatment planning system (TPS), and each is explained in order. As the key points for MU manual calculation, this paper explained (1) tissue phantom dose ratio and tissue maximum dose ratio, (2) total scattering coefficient, (3) collimator scattering coefficient, (4) off-axis dose ratio, and (5) conversion to equivalent irradiation field. In reality, it is desirable to construct a verification program that includes an exclusive use of software when installing the device. MU verification requires two independences: independence of an MU verification implementation program and implementation of MU verification by a person in charge different from the planner. (A.O.)