To avoid the excessive suppression of electricity demand caused by conservative planning of electricity price, real-time minute-to-minute control of the electricity price, which is called real-time pricing, is required. However, the electricity demand response has the dynamics on a timescale shorter than an hour, which can cause catastrophic incidents if the real-time pricing is done without ample consideration, and the model for such dynamics is demanded. For this background, a new dynamic model for the electricity demand response is proposed in this paper. Since the main source of the price elasticity utilized in realtime pricing is considered to be the temperature setting of HVAC equipment, an experiment with an air conditioner is performed to get a quick sketch of the dynamics. Then, from the experimental results and knowledge from existing literatures, a kind of dynamic linear model with Markov-switching is introduced and shown to be a good model for the electricity demand response on a timescale shorter than an hour.