Because reaction force estimation used in teleoperated robots is exposed to threats of cyberattacks, ensuring cyber security of reaction force estimation is important. The purpose of this study is to develop an encrypted reaction force observer. The state-space representation of the reaction force observer is organized and represented as a product of a matrix and a vector. ElGamal encryption, which can compute the product of ciphertexts, is applied to the calculation of the reaction force observer. The results of the reaction force estimation experiment show that the reaction force observer with encrypted control can estimate the reaction force.