A battery state judging method is provided capable of properly judging a state of a lead-acid battery by measuring voltages without measuring currents. A state of the battery is judged by obtaining, in advance, a DC internal resistance r0 of a same type battery in a new and fully-charged state; calculating, in advance, a load R [R=r0×Vst0/(OCV0−Vst0)] at an engine starting time (S112) from an open-circuit voltage OCV0 (S104) and a lowest voltage Vst0 (S108) at the engine starting time, both measured when the loaded battery is in a new and fully-charged state, and the DC internal resistance r0; calculating a DC internal resistance r [(r=R/(OCV−Vst)/Vst)] of the loaded battery from an open-circuit voltage OCV (S104) and a lowest voltage Vst (S108), both measured when the loaded battery is in a deteriorated/insufficiently-charged state, and the load R; and comparing r and r0.