A container assembly has a raw-material container and a product-collection container, which are heat resistant and pressure resistant and have graduations formed on a sidewall thereof and a joint protruding therefrom. The product-collection container is detachably mounted on the raw-material container and communicates with the raw-material container through the joint. Since the raw-material container has graduations, an amount of raw materials can be consistently added in each batch, therefore, conditions of sublimation such as pressure, temperature or the like do not require adjustment and may just be monitored to ensure maximum yield is attained. Therefore, a sublimation procedure is simple, saves time and decreases product costs. Since, the product-collection container has graduations, an amount of product can be observed easily by the graduations and the product is easily removed without removing impure byproducts. Therefore, purity of the product can be increased.