In-situ observations of an ice crystal growing or melting in trehalose or sucrose solutions of various concentrations were performed to study the concentration dependence of melting point of the ice crystal in each solution. Melting point of the ice crystal in trehalose solution was identical with that in sucrose solution. The experimental values of melting points agreed with the formula of the molar depression of freezing point that is applicable in low concentration aqueous solutions when free water was expected to be remaining. Above the molality to be estimated that no free water exist, melting point of ice crystal was lower than the formula of the molar depression of freezing point.
