To provide an electronic computer for checking multiplication including at least one factor with two or more digits.

Caluculation is performed based on a program to which the law of a compression number has been input. As for the law of a compression number, when the addition of a number at each digit is defined as compression calculation, and a value obtained by operating compression calculation is defined as a compression number, and a value obtained by repeatedly operating compression calculation several times until a compression number with two or more digits is reduced to one digit is defined as one digit compression number, one digit compression number of a factor with two or more digits or one digit compression number of the compression factor product obtained by multiplying factors with one digit is the same value as one digit compression number of the product obtained by multiplying original factors as they are. When any result which is contradictory to the law of a compression number is calculated, it is decided that the answer of multiplication is not correct, and when the result is not contradictory to the law of a compression number, the correctness of the answer of multiplication is not decidable.