There is provided a biological state monitoring system for monitoring biological state of subject on a bed. The system includes at least one load detector and a controller. In a case that the controller determines that body motion is not twitch, the controller ceases to output estimated value of respiratory rate, or outputs first estimated value which is the latest among estimated values obtained in first period, the first period being directly preceding body motion and being determined by the controller to be a period in which subject has no body motion. In a case that the controller determines that body motion is twitch, the controller successively outputs newly obtained estimated values of respiratory rate.