The number of minors victimized in such cases came to 638, also hitting an all-time high, the report said.
The numbers of child porn cases uncovered and victims increased 8.4 percent and 3.9 percent, respectively, from the previous record highs of 1,342 and 618, both in 2010.
Of the 638 victims, elementary school students accounted for 85 and preschool children 20. A 3-year-old girl was the youngest among the victims.
The police could not identify the victims in 648 images, of which 570 or nearly 90 percent may be preschool children, the report said.
The number of child porn cases that used file-sharing programs for delivery increased 136 percent from 2010 to 368.
Last year, major Japanese Internet service providers began blocking access to websites that offer child pornography, leading to the sharp increase in the number of cases involving use of file-sharing programs, the police agency said.
Meanwhile, the police uncovered 384 child abuse cases in 2011, an increase of 9.1 percent from the preceding year, and arrested or questioned 409 people on suspicion of abusing children, up 6.2 percent.
The number of children abused last year came to 398, an increase of 36 or 10 percent from the 362 in 2010. Of the victims, 39 died, up from 33 the previous year.
The number of people aged under 20 who were accused in criminal cases came to 77,696 in 2011, down 9.4 percent from 2010 for the eighth straight year-on-year decline, the report said.