A therapeutic agent comprising: a δ-aminolevulinic acid represented by the formula, a derivative thereof, or a salt thereof; and an iron compound: R 2 R 1 NCH 2 COCH 2 CH 2 COR 3 wherein R 1 and R 2 each independently represents a hydrogen atom, an alkyl group, an acyl group, an alkoxycarbonyl group, an aryl group, or an aralkyl group; and R 3 represents a hydroxy group, an alkoxy group, an acyloxy group, an alkoxycarbonyloxy group, an aryloxy group, an aralkyloxy group, or an amino group; wherein the derivative of the δ-aminolevulinic acid of formula is selected from δ-aminolevulinic acid methyl ester, δ-aminolevulinic acid ethyl ester, δ-aminolevulinic acid propyl ester, δ-aminolevulinic acid butyl ester, δ-aminolevulinic acid pentyl ester and δ-aminolevulinic acid hexyl ester; wherein the therapeutic agent is for use in (i) a method for treating a mitochondrial dysfunction brain disease, wherein the mitochondrial dysfunction brain disease is selected from amyotrophic lateral sclerosis, mitochondria myopathy, migraine, Parkinson's disease, hypoxic encephalopathy, cerebral arteriosclerosis, manic-depressive psychosis, chronic fatigue syndrome, intracranial hypertension, normal pressure hydrocephalus, cerebral vasospasm after subarachnoid hemorrhage, atherothrombotic cerebral infarction, lacunar infarction, cardiogenic cerebral embolism, or Alzheimer's disease; or (ii) a method for the prevention of a mitochondrial dysfunction brain disease, wherein the mitochondrial dysfunction brain disease is selected from cerebral ischemia at surgery and intravascular surgery. a δ-aminolevulinic acid represented by the formula, a derivative thereof, or a salt thereof; and an iron compound: R 2 R 1 NCH 2 COCH 2 CH 2 COR 3 wherein R 1 and R 2 each independently represents a hydrogen atom, an alkyl group, an acyl group, an alkoxycarbonyl group, an aryl group, or an aralkyl group; and R 3 represents a hydroxy group, an alkoxy group, an acyloxy group, an alkoxycarbonyloxy group, an aryloxy group, an aralkyloxy group, or an amino group; wherein the derivative of the δ-aminolevulinic acid of formula is selected from δ-aminolevulinic acid methyl ester, δ-aminolevulinic acid ethyl ester, δ-aminolevulinic acid propyl ester, δ-aminolevulinic acid butyl ester, δ-aminolevulinic acid pentyl ester and δ-aminolevulinic acid hexyl ester; wherein the therapeutic agent is for use in (i) a method for treating a mitochondrial dysfunction brain disease, wherein the mitochondrial dysfunction brain disease is selected from amyotrophic lateral sclerosis, mitochondria myopathy, migraine, Parkinson's disease, hypoxic encephalopathy, cerebral arteriosclerosis, manic-depressive psychosis, chronic fatigue syndrome, intracranial hypertension, normal pressure hydrocephalus, cerebral vasospasm after subarachnoid hemorrhage, atherothrombotic cerebral infarction, lacunar infarction, cardiogenic cerebral embolism, or Alzheimer's disease; or (ii) a method for the prevention of a mitochondrial dysfunction brain disease, wherein the mitochondrial dysfunction brain disease is selected from cerebral ischemia at surgery and intravascular surgery. wherein R 1 and R 2 each independently represents a hydrogen atom, an alkyl group, an acyl group, an alkoxycarbonyl group, an aryl group, or an aralkyl group; and R 3 represents a hydroxy group, an alkoxy group, an acyloxy group, an alkoxycarbonyloxy group, an aryloxy group, an aralkyloxy group, or an amino group; wherein the derivative of the δ-aminolevulinic acid of formula is selected from δ-aminolevulinic acid methyl ester, δ-aminolevulinic acid ethyl ester, δ-aminolevulinic acid propyl ester, δ-aminolevulinic acid butyl ester, δ-aminolevulinic acid pentyl ester and δ-aminolevulinic acid hexyl ester; wherein the therapeutic agent is for use in (i) a method for treating a mitochondrial dysfunction brain disease, wherein the mitochondrial dysfunction brain disease is selected from amyotrophic lateral sclerosis, mitochondria myopathy, migraine, Parkinson's disease, hypoxic encephalopathy, cerebral arteriosclerosis, manic-depressive psychosis, chronic fatigue syndrome, intracranial hypertension, normal pressure hydrocephalus, cerebral vasospasm after subarachnoid hemorrhage, atherothrombotic cerebral infarction, lacunar infarction, cardiogenic cerebral embolism, or Alzheimer's disease; or (ii) a method for the prevention of a mitochondrial dysfunction brain disease, wherein the mitochondrial dysfunction brain disease is selected from cerebral ischemia at surgery and intravascular surgery. (i) a method for treating a mitochondrial dysfunction brain disease, wherein the mitochondrial dysfunction brain disease is selected from amyotrophic lateral sclerosis, mitochondria myopathy, migraine, Parkinson's disease, hypoxic encephalopathy, cerebral arteriosclerosis, manic-depressive psychosis, chronic fatigue syndrome, intracranial hypertension, normal pressure hydrocephalus, cerebral vasospasm after subarachnoid hemorrhage, atherothrombotic cerebral infarction, lacunar infarction, cardiogenic cerebral embolism, or Alzheimer's disease; or (ii) a method for the prevention of a mitochondrial dysfunction brain disease, wherein the mitochondrial dysfunction brain disease is selected from cerebral ischemia at surgery and intravascular surgery.