PURPOSE: To easily calculate a specific calendar day of desired kind without being given data on the perpetual calendar by others.

CONSTITUTION: This device is equipped with a data and formula storage part 2 stored with a group of data regarding heavenly body tracks and calendar days and a group of formulas including respective heavenly body track calculating formulas and respective calendar day information calculating formulas, an input part 3 for inputting requested year, month, and day information and calendar day kind information, a calendar day calculating process part 1, and an output part 4 which outputs a calculated calendar day. The calendar day calculating process part 1 calculates the Julian day by using corresponding data and a formula in the data group and formula group on the basis of the requested year, month, and day information from the input part 3, discriminates requested calendar day kind information, and selects data and a formula corresponding to the discriminated calendar day kind to calculate the calendar day corresponding to the Julian day.