An ion current detection system (80, 80B) for detecting current flowing as a result of generation of ions in a combustion chamber of an internal combustion engine, comprising: an ignition assist device attached to the internal combustion engine and comprising an ion detection electrode and a heat-generating element which are embedded in a ceramic member in such a manner as to be electrically insulated from each other, an ion detection terminal portion for applying an ion detection voltage to the ion detection electrode, and two heat-generating-element terminal portions (21, 22) adapted to supply electricity to the heat-generating element; an ion current detection circuit comprising a first terminal electrically connected to the ion detection terminal portion and a second terminal electrically connected to the internal combustion engine; and a heat-generating-element circuit (70, 70B) comprising a first terminal electrically connected to one of the two heat-generating-element terminal portions and a second terminal electrically connected to the other one of the two heat-generating-element terminal portions; characterized in that: the two heat-generating-element terminal portions (21, 22) are disposed at such positions as to not come into contact with the internal combustion engine, and by a switching element for electrically connecting the second terminal of the heat-generating-element circuit and the internal combustion engine to each other or electrically disconnecting the second terminal of the heat-generating-element circuit and the internal combustion engine from each other, wherein at least in detection of ion current by the ion current detection circuit, the switching element electrically disconnects the internal combustion engine and the second terminal of the heat-generating-element circuit from each other. an ignition assist device attached to the internal combustion engine and comprising an ion detection electrode and a heat-generating element which are embedded in a ceramic member in such a manner as to be electrically insulated from each other, an ion detection terminal portion for applying an ion detection voltage to the ion detection electrode, and two heat-generating-element terminal portions (21, 22) adapted to supply electricity to the heat-generating element; an ion detection electrode and a heat-generating element which are embedded in a ceramic member in such a manner as to be electrically insulated from each other, an ion detection terminal portion for applying an ion detection voltage to the ion detection electrode, and two heat-generating-element terminal portions (21, 22) adapted to supply electricity to the heat-generating element; an ion current detection circuit comprising a first terminal electrically connected to the ion detection terminal portion and a second terminal electrically connected to the internal combustion engine; and a first terminal electrically connected to the ion detection terminal portion and a second terminal electrically connected to the internal combustion engine; and a heat-generating-element circuit (70, 70B) comprising a first terminal electrically connected to one of the two heat-generating-element terminal portions and a second terminal electrically connected to the other one of the two heat-generating-element terminal portions; characterized in that: the two heat-generating-element terminal portions (21, 22) are disposed at such positions as to not come into contact with the internal combustion engine, and by a switching element for electrically connecting the second terminal of the heat-generating-element circuit and the internal combustion engine to each other or electrically disconnecting the second terminal of the heat-generating-element circuit and the internal combustion engine from each other, wherein at least in detection of ion current by the ion current detection circuit, the switching element electrically disconnects the internal combustion engine and the second terminal of the heat-generating-element circuit from each other. a first terminal electrically connected to one of the two heat-generating-element terminal portions and a second terminal electrically connected to the other one of the two heat-generating-element terminal portions; characterized in that: the two heat-generating-element terminal portions (21, 22) are disposed at such positions as to not come into contact with the internal combustion engine, and by a switching element for electrically connecting the second terminal of the heat-generating-element circuit and the internal combustion engine to each other or electrically disconnecting the second terminal of the heat-generating-element circuit and the internal combustion engine from each other, wherein at least in detection of ion current by the ion current detection circuit, the switching element electrically disconnects the internal combustion engine and the second terminal of the heat-generating-element circuit from each other. the two heat-generating-element terminal portions (21, 22) are disposed at such positions as to not come into contact with the internal combustion engine, and by a switching element for electrically connecting the second terminal of the heat-generating-element circuit and the internal combustion engine to each other or electrically disconnecting the second terminal of the heat-generating-element circuit and the internal combustion engine from each other, wherein at least in detection of ion current by the ion current detection circuit, the switching element electrically disconnects the internal combustion engine and the second terminal of the heat-generating-element circuit from each other.