A glow plug deterioration determination system 1 includes: a glow plug 40; a GCU 30 that controls energization of the glow plug 40 from a power supply 10 by driving switch means T₁ to T₄ to open and close in accordance with a drive signal SI issued by an ECU 20; a self-diagnosis unit (DIU) 32 that detects an abnormality between the glow plug 40, the switch means T₁ to T₄, and the GCU 30 and transmits a self-diagnosis signal DI to the ECU 20; and current detecting means S₁ to S₄ for detecting a plug current I_GL flowing through the glow plug 40, wherein the DIU 32 includes deterioration level determining means 330 for determining whether the glow plug 40 is in a deteriorated condition by comparing a plug current converted voltage Vi that is proportionate to the plug current I_GL with a plurality of voltage thresholds Vref₁ to Veef_n.