To provide a battery charging controller which can prevent a main battery power from being consumed due to execution of charging of a sub battery, under the condition of the charging of the main battery being insufficient.

A switch 21 for current application control is interposed between the main battery 5, which is charged by the rectified output of a generator 1 driven by an engine, and the sub battery 6. This battery charging controller charges the sub battery 6, giving a drive signal to the switch 21 for current application control, only when it is confirmed that the main battery voltage is at a value capable of permitting the charging of the sub battery 6, and that the rotational speed of the engine is at or over a value required for generating an output sufficient for enabling the charging of both of the main battery and the sub battery 6 from the generator 1, and moreover when the voltage of the sub battery needs to be charged.