The ventilation efficiency of an artificially controlled experimental piglet house was evaluated using the tracer gas decay (TGD) method, which can predict the airflow pattern by tracking the tracer gas. The ventilation rate of the exhausted fan was classified as 120, 80, and 40 m³ min⁻¹. Three inlet types were used as follows: (1) side windows, (2) ceiling slot with opening angle of 90°, and (3) ceiling slot with opening angle of 45°. Then, the local mean age (LMA) and air supply effectiveness (ASE) were calculated from the measured data at 18 points (3 × 3 sensors placed top and bottom). The LMA means the time for the inflow air to reach a point and ASE is the ratio of the LMA at a point to the total average LMA in the facility. When side windows and ceiling slots were used with the ventilation rate of 120 m³ min⁻¹, each ASE was approximately 0.8 and 0.9, respectively, suggesting that ceiling slots could be more efficient than side windows with high ventilation rate. When the ventilation rate was low, the ASE of side windows was higher than the ceiling slots. However, when the angle of ceiling slots was changed to 45° with the ventilation rate of 40 m³ min⁻¹, the ASE was almost 1.0, indicating that the ventilation efficiency was improved. Thus, ventilation efficiency can be sufficiently improved for all ventilation rates by using ceiling slots whilst controlling the angle of the slots.