Provided is a compound and composition capable of increasing a sugar content in a fruit by a simple method, without being restricted by a cultivation area of a plant or a climatic environment. The agent for increasing a sugar content in a fruit of a plant comprises a compound represented by the formula MX as an active ingredient, wherein M represents alkali metal ion or alkaline earth metal ion, and X represents carbonate ion, hydrogen carbonate ion, acetate ion, citrate ion, succinate ion, phosphate ion, hydrogen phosphate ion, or pyrophosphate ion.