It is possible to provide a heat storage substance, a heat storage agent, a heat transport medium, and a cold insulation agent having low corrosiveness and a high latent heat quantity at a low cost, their production method, and a cold reservoir and a cold insulator containing the heat storage substance as content. The heat storage substance is characterized by containing a tri-n-butylalkylammonium salt and water. The heat storage substance is characterized by containing either tri-n-butyl-n-pentylammonium bromide or tri-n-butyl-n-pentylammonium chloride and water. The heat storage agent is characterized by containing a tri-n-butylalkylammonium salt and water. The heat storage agent is characterized by containing either tri-n-butyl-n-pentylammonium bromide or tri-n-butyl-n-pentylammonium chloride and water. The heat transport medium is characterized by containing a tri-n-butylalkylammonium salt and water. The heat transport medium is characterized by containing either tri-n-butyl-n-pentylammonium bromide or tri-n-butyl-n-pentylammonium chloride and water.
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