A chemical heat storage device includes a reactor disposed around an oxidizing catalyst and containing MgCl2 which chemically reacts with NH3 to generate heat, a storage connected to the reactor through a pipe to store NH3, an on-off valve disposed in the pipe, a temperature sensor detecting the temperature of the exhaust gas passing through the oxidizing catalyst, and a controller controlling the on-off valve based on the detected value of the temperature sensor. The controller controls the on-off valve such that the on-off valve unconditionally opens if the temperature of the exhaust gas is greater than a heat generating start temperature TL and is equal to or less than a heat generating end guide temperature TQ, and the on-off valve unconditionally opens if the temperature of the exhaust gas is greater than a regenerating temperature TH.