A 4th generation synchrotron radiation light source for a large-scale user facility like the 3rd ones has been recently discussed to realize using a superconducting RF linac based X-ray free electron laser (FEL) [1] or an energy recovery superconducting linac (ERL) driven light source [2]. In order to realize both of them, we need very large number of low-temperature cryostats to run them routinely and efficiently. A half kilomter-long superconducting RF linac cryostat is conceptually considered here and discussed to optimize major operational characteristics of the advanced lowtemperature cryostat for the ERL light source near future. In addition to them, we summarize the JAERI ERL FEL zero-boil off cryostat design, its operational characteristics, and non-stop operation records over these 13 years and other cryostat related activities. CRYOSTAT CONSIDERATION FOR INDUSTRIAL ERL LIGHT SOURCES In order to realize quickly an industrial energy recovery linac (ERL) light source, a half kilomter-long superconducting RF linac cryostat is conceptually designed and discussed to optimize major operational characteristics of the advanced low-temperature cryostat for an ERL light source near future. A low temperature cryostat for superconducting RF linacs and other superconducting devices should be designed and made to minimize its heat invasion through the heat bridges between the room and low temperature parts. The cryostat also should be designed and made to minimize total number of the heat cycles for the rest of its life after the initial test. We can expect that the total number of the heat bridges is roughly proportional to the number of the cryostat, and a life interval between two contiguous cryostat malfunctions inversely proportional to the number of the heat cycles. Therefore, if we can design and make a single and long cryostat to realize the operation less than a half cycle for the rest of its life, we can run our cryostat for a very longer interval than tens of years and with minimum electricity. In addition to them, we plan to summarize the JAERI ERL FEL zero-boil off cryostat design, its operational characteristics, and nonstop operation records over these 12 years and other cryostat related activities in the poster presentation. Cryostat Heat Load