We report here the results of Debye-Scherrer X-ray diffraction measurements in the temperature range from 12K to 300K of the perovskite compounds SrRuO
3 and CaRuO
3. We show that SrRuO
3 indicates an clear anomaly with the temperature dependence of the unit-cell parameters and volume below its ferromagnetic transition temperature T
c of 160K through magnetovolume effect. In particular, the volume of SrRuO
3 is almost constant below T
c, and invar effect is discovered for the first time in transition-metal oxide, which is seen in 3d transition-metal itinerant magnets, e.g., Fe-Ni alloy. Therefore, it is concluded that SrRuO
3 is classified to a moderately weak itinerant ferromagnet. On the other hand, CaRuO
3 shows no anomaly in the variation of the volume with temperature, suggesting that magnetic ordering does not occur in this compound.
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