A fiber diameter measuring device, an inorganic fiber sheet producing device, a method for measuring a fiber diameter, and a method for producing an inorganic fiber sheet, capable of guaranteeing continuous safeness of the fiber diameter of an inorganic fiber in an inorganic fiber sheet is provided. A method further capable of measuring the fiber diameter of the inorganic fiber in the inorganic fiber sheet, not only after the production of the inorganic fiber sheet but also during the production of the inorganic fiber sheet is provided. The fiber diameter measuring device includes a sampling section capable of sampling a predetermined amount of the inorganic fiber in an entire area of the inorganic fiber sheet; and a measuring section configured to measure a fiber diameter of the sampled inorganic fiber.