An apparatus for holding a test tube in which insulated isothermal polymerase chain reaction is performed includes a heat insulating mount and a heating member. The heat insulating mount has a main body provided with a receiving space for receiving a bottom of the test tube, a lateral channel communicated between the receiving space and an ambient environment, and an upper channel communicated between the receiving space and the ambient environment for insertion of the test tube. The heating member is inserted into the lateral channel for stopping at the bottom of the test tube. The apparatus can minimize the influence caused by the high temperature generated from the heat source on the heat dissipation of the reaction mixture in middle and upper sections of the test tube. The apparatus is suitable for the fluorescent detection of PCR reaction.