[Problem] To provide a magnesium hydroxide-based solid solution which has smaller primary particles and secondary particles and improved reactivity with an acid as compared with conventional magnesium hydroxide (Mg(OH)2), improves the flame retardancy and mechanical strength of a resin, and also forms a non-sedimenting slurry, providing the same handleability as a liquid.[Structure] A magnesium hydroxide-based solid solution represented by the following formula (1): Mg(OH)2-xRx (Formula 1), wherein R represents a monovalent organic acid, and x represents 0