The TGA-51, a new macro-TG which can detect slight weight changes (mu g order) in samples of 10 g was developed. As one of applications using this TG, a weight change of Pt-loaded tungsten(VI) oxide in H-2 atmosphere was measured. It is well known that the sample forms hydrogen insertion compounds HxWO3 (x = 0.3-0.4) at ambient temperature by hydrogen spillover. Above 373 K, the sample showed a period of delayed reaction of approximately 30 min before the hydrogen insertion and the value of x was lower than that observed at room temperature.