PURPOSE: To know exactly a strength appearing state of a sample by putting the sample of mass concrete into a water tank, and controlling a temperature of the water tank to match a history temperature of mimic mass concrete.

CONSTITUTION: A sample S of mass concrete is contained in the water of a water tank 1. Also, a heating unit 2 is provided in this water tank 1, and controlled by a temperature controller 4 in a control box 2. A thermocouple 5 is connected as a temperature detector to the temperature controller 4, installed to a suitble place P1 of actual construction mass concrete, or a suitable place P2 of mimic mass concrete imitated to its actual construction mass concrete, and a history temperature of its installed place is detected. Also, the temperature controller 4 controls the heating unit 2 so that a water temperature of the water tank 1 is matched to a detected temperature of the thermocouple 5.