To provide: sheet-shaped cerium oxide having an average particle diameter of less than 20 [mu]m； a petal-shaped cerium oxide powder which is an aggregate of the sheet-shaped cerium oxide； a process for producing the sheet-shaped cerium oxide and the petal-shaped cerium oxide powder； and a cosmetic which is produced by adding the sheet-shaped cerium oxide or the petal-shaped cerium oxide powder which is an aggregate of the sheet-shaped cerium oxide to a cosmetic to thereby impart a high ultraviolet ray blocking effect and an excellent feeling upon application to the cosmetic. [Solution] Sheet-shaped cerium oxide having an average particle diameter of less than 20 [mu]m and an average aspect ratio of 2-80 and a petal-shaped cerium oxide powder which is an aggregate of the sheet-shaped cerium oxide can be produced by preparing an aqueous solution comprising cerium nitrate or cerium chloride, sodium hydrogen carbonate or sodium carbonate and water so that the molar ratio of carbonate ions to cerium ions falls within the range of 1.5-5, maintaining the aqueous solution at a temperature falling within the range of 0-40 DEG C to cause the precipitation of cerium carbonate particles from the aqueous solution, and then burning the cerium carbonate particles.本發明之目的在於提供一種平均粒徑為未達20μm之板狀氧化鈰及其集合體之花瓣狀氧化鈰粉體與其製造方法，且藉由將其板狀氧化鈰及其集合體之花瓣狀氧化鈰粉體摻混於化妝料，可提供一種紫外線遮蔽效果高、且使用感優異之化妝料；藉由將含有硝酸鈰或氯化鈰、碳酸氫鈉或碳酸鈉、與水之溶液，調製成碳酸離子對鈰離子之莫耳比為1.5～5的範圍內，使該水溶液的溫度維持在0～40℃之範圍內，從該水溶液析出碳酸鈰粒子，進一步地藉由燒成，可得到平均粒徑為未達20μm且平均長寬比為2～80之板狀氧化鈰及其集合體之花瓣狀氧化鈰粉體。
