In an internal combustion engine which includes a combustion pressure sensor incorporated glow plug and sensor nonincorporated glow plugs, a first plug regression line Lp1 and the second plug regression line Lp2 are in such a relation that, at any temperature Tx within the temperature range Tr, a second voltage Vp2x is lower than a first voltage Vp1x; the resistance of a second electricity supply line is greater than the resistance of a first electricity supply line; and a first section regression line and a second section regression line are in such a relation that, at any temperature Tx within the temperature range Tr, an overall voltage deviation |Vc1x−Vc2x|, which is the absolute value of the difference between a third voltage Vc1x and a fourth voltage Vc2x, is smaller than a first-second plug voltage difference (Vp1x−Vp2x).