A throttle controller that controls an opening degree of a throttle valve installed in an intake passage of an engine, wherein an opening degree of the throttle valve is defined as a throttle opening TA, a target value of the throttle opening degree TA is defined as a target opening degree TA*, a maximum value in a control range of the throttle opening degree TA is defined as a maximum opening degree TAmax, a required value of a load factor KL of the engine is defined as a required load factor KL*, a maximum value of the load factor KL in a current controlled state of the engine is defined as a maximum load factor KLmax, a ratio of an intake pressure after passing through the throttle valve to an intake pressure before passing through the throttle valve is defined as a throttle upstream-downstream pressure ratio RP, the throttle upstream-downstream pressure ratio RP required to cause the load factor KL to be the required load factor KL* is defined as a required pressure ratio RP*, the throttle opening degree TA when the throttle upstream-downstream pressure ratio RP is a specified value RPwot is defined as a switching point opening degree TAwot, the load factor KL when the throttle upstream-downstream pressure ratio RP is the specified value RPwot is defined as a switching point load factor KLwot, the throttle controller is configured to execute a target opening degree calculation process and a throttle actuation process, in the target opening degree calculation process, the throttle controller calculates, as the target opening degree TA*, the throttle opening degree TA at which the throttle upstream-downstream pressure ratio RP is the required pressure ratio RP* in a case in which the value of the required pressure ratio RP* is less than or equal to the specified value RPwot, and calculates, as the target opening degree TA*, a value that satisfies a following expression in a case in which the value of the required pressure ratio RP* exceeds the specified value RPwot, and TA * = TAwot + TAmax − TAwot × KL * − KLwot KLmax − KLwot in the throttle actuation process, the throttle controller actuates throttle valve to cause the throttle opening degree TA to be the target opening degree TA*. an opening degree of the throttle valve is defined as a throttle opening TA, a target value of the throttle opening degree TA is defined as a target opening degree TA*, a maximum value in a control range of the throttle opening degree TA is defined as a maximum opening degree TAmax, a required value of a load factor KL of the engine is defined as a required load factor KL*, a maximum value of the load factor KL in a current controlled state of the engine is defined as a maximum load factor KLmax, a ratio of an intake pressure after passing through the throttle valve to an intake pressure before passing through the throttle valve is defined as a throttle upstream-downstream pressure ratio RP, the throttle upstream-downstream pressure ratio RP required to cause the load factor KL to be the required load factor KL* is defined as a required pressure ratio RP*, the throttle opening degree TA when the throttle upstream-downstream pressure ratio RP is a specified value RPwot is defined as a switching point opening degree TAwot, the load factor KL when the throttle upstream-downstream pressure ratio RP is the specified value RPwot is defined as a switching point load factor KLwot, the throttle controller is configured to execute a target opening degree calculation process and a throttle actuation process, in the target opening degree calculation process, the throttle controller calculates, as the target opening degree TA*, the throttle opening degree TA at which the throttle upstream-downstream pressure ratio RP is the required pressure ratio RP* in a case in which the value of the required pressure ratio RP* is less than or equal to the specified value RPwot, and calculates, as the target opening degree TA*, a value that satisfies a following expression in a case in which the value of the required pressure ratio RP* exceeds the specified value RPwot, and TA * = TAwot + TAmax − TAwot × KL * − KLwot KLmax − KLwot calculates, as the target opening degree TA*, the throttle opening degree TA at which the throttle upstream-downstream pressure ratio RP is the required pressure ratio RP* in a case in which the value of the required pressure ratio RP* is less than or equal to the specified value RPwot, and calculates, as the target opening degree TA*, a value that satisfies a following expression in a case in which the value of the required pressure ratio RP* exceeds the specified value RPwot, and TA * = TAwot + TAmax − TAwot × KL * − KLwot KLmax − KLwot in the throttle actuation process, the throttle controller actuates throttle valve to cause the throttle opening degree TA to be the target opening degree TA*.