A decrease in the accuracy after switching of the heating state can be reduced. The physical quantity detecting device includes: a flow rate detecting unit configured to include a heating element to measure a flow rate of a fluid to be measured; a heating element control unit configured to switch a control state of the heating element to any one of a heating state and a heating suppression state; and a signal processing unit configured to process a measured value of the flow rate detecting unit. When the heating element control unit switches the control state, the signal processing unit processes an estimated value determined based on a measured value of the flow rate detecting unit before the switching for a predetermined period immediately after the switching.