Alumina cement mortar is used as a source material for a kneaded cement cured body for joining hardware onto an insulator body, whereby the insulator can maintain high initial and long term mechanical strength and high electrical strength over a long period of time, and the curing time for forming the cured body is shortened to reduce the cost of producing the insulator. The alumina cement mortar is formed by kneading a mixture of alumina cement particles having a specific surface area of at least 3500 cm2/g, a polymer-steric-hindrance type water reducing agent, an aggregate, and water, or an alumina cement mortar is obtained by kneading a mixture of alumina cement particles having an amorphous phase on an outer peripheral surface of particles and having a specific surface area of at least 3500 cm2/g, a water reducing agent, an aggregate, and water.