This invention is to provide a diffusion testing device to facilitate determination of diffusivity of prolonged release of drugs from transdermal therapeutic preparation.The diffusion testing device is characterized by a receptor liquid reservoir; a diffusion testing cell having a receptor chamber functioning to be full of receptor liquid and within which a stirrer be placed, a receptor liquid supply unit, a thermostat bath to hold the diffusion testing cell, a stirring apparatus located beneath the thermostat bath to cause the rotation action of the stirrer set in the receptor chamber, a sampling unit permitting sampling and storage of receptor liquid dissolving permeated drugs. Said receptor liquid reservoir, said receptor chamber, said receptor liquid supply unit and said sampling unit are connected with each other via infusion tube. Said thermostat bath comprises aperturees or through-holes functioning to hold the diffusion testing cells, and metal block which has temperature regulation unit to be installed.