A secondary battery state detection device for detecting a state of a secondary battery, comprising: one or a plurality of processors; and one or a plurality of memories (10b, 10c) that are communicably connected to said one or a plurality of processors, wherein said one or a plurality of processors is configured to read a set of instructions stored in said one or a plurality of memories (10b, 10c), and the at least one processor is further configured to execute: a detection process that detects a charging voltage of an alternator when the alternator charges the secondary battery, a voltage generated by the alternator being adjustable, a computation process that calculates a charging resistance as an internal resistance of the secondary battery while the alternator is charging the secondary battery, and an estimation process that estimates a state of charge of the secondary battery on the basis of the charging voltage detected by the detection process and the charging resistance calculated by the computation process, and characterized in that in the estimation process the secondary battery is estimated to have a predetermined state of charge, if a value of the charging resistance calculated by the computation process is greater than or equal to a predetermined threshold. one or a plurality of processors; and one or a plurality of memories (10b, 10c) that are communicably connected to said one or a plurality of processors, wherein said one or a plurality of processors is configured to read a set of instructions stored in said one or a plurality of memories (10b, 10c), and the at least one processor is further configured to execute: a detection process that detects a charging voltage of an alternator when the alternator charges the secondary battery, a voltage generated by the alternator being adjustable, a computation process that calculates a charging resistance as an internal resistance of the secondary battery while the alternator is charging the secondary battery, and an estimation process that estimates a state of charge of the secondary battery on the basis of the charging voltage detected by the detection process and the charging resistance calculated by the computation process, and characterized in that in the estimation process the secondary battery is estimated to have a predetermined state of charge, if a value of the charging resistance calculated by the computation process is greater than or equal to a predetermined threshold. a detection process that detects a charging voltage of an alternator when the alternator charges the secondary battery, a voltage generated by the alternator being adjustable, a computation process that calculates a charging resistance as an internal resistance of the secondary battery while the alternator is charging the secondary battery, and an estimation process that estimates a state of charge of the secondary battery on the basis of the charging voltage detected by the detection process and the charging resistance calculated by the computation process, and characterized in that in the estimation process the secondary battery is estimated to have a predetermined state of charge, if a value of the charging resistance calculated by the computation process is greater than or equal to a predetermined threshold.