A horizontal dilution refrigerator was constructed for a spin frozen proton and deuteron target at KEK (National Laboratory for High Energy Physics). It was designed to obtain very high cooling capacity over the wide temperature range, especially at rather high temperatures (50 mK ∼ 500 mK). It has a high gas flow rate (typically 4.5 × 10 −3 mol s −1), a powerful 4He precooling system and an adequate heat exchange efficiency between the incoming stream and outgoing solution. It has double-tube heat exchangers, whose lower temperature parts are filled with sintered copper. The mixing chamber is made of PFA which is joined with a brass flange by a simple demountable seal. Typical cooling power was about 40 mW and 5 mW at 500mK and 200 mK respectively and the lowest temperature was about 20 mK.