Provided are a radiation dosimeter for fluid very small substances, which, in a radiation irradiation device for applying a radiation to a fluid very small substance in a fluid in a treatment chamber, can easily and accurately determine a dose distribution and/or minimum dose of a radiation applied to individual fluid very small substances, and a method of measuring a dose of a radiation applied to fluid very small substances using the dosimeter. In the radiation dosimeter for fluid very small substances, a microcapsule includes a radiation transmitting shell body and a radiation color development tautomeric photochromic solution contained within the shell. The microcapsule undergoes a change in color upon a change in color of the photochromic solution upon exposure to a radiation. The dose of the radiation and the color change level of the microcapsule have a quantitative relationship. Further, the particle diameter, which has a peak value in the particle diameter distribution of the microcapsule, is set to substantially the same diameter as the fluid very small substance as a dose measurement object. In the method of measuring the dose of a radiation applied to fluid very small substances, the radiation dosimeter for fluid very small substances is used.