[en] Design, dimensions and usage of capsules and holders used for irradiation of target materials to produce radioisotopes in reactors in the Japan Atomic Energy Research Institute are described. The capsule consists of an inner capsule to contain a target material and an outer capsule to enclose the inner one. Organic polymers such as polyethylene and polyimide, quartz, aluminum and stainless steel are used for capsule materials. Design and dimensions of the capsules are various. A combination of the inner and the outer capsule of different designs, dimensions and materials is used depending on the kind, properties and quantity of a target material for irradiation and the irradiation conditions. To irradiate mercury, sulfur, uranium and iridium, the respective capsules are used. Gastight capsules are used for irradiation of those materials releasing radioactive gases. Aluminum capsules as such are sealed hermetically by cold or arcwelding. The capsules are then inspected in gastightness by bubbling test in ethyleneglycol or helium leak detector. Results of various tests carried out in development of gastight and pressure resistant capsules are given. In the past ten years, breakage occurred only in one gastight capsule. This situation and results of the investigation for its cause are briefed. (author)