Disclosed is a kit for hair dyeing or bleaching which includes: a first part containing an alkali agent; a second part containing an oxidizing agent; and an openable and closable airtight container into which the first and second parts are charged so as to form foam of a mixture thereof. Also disclosed is a method for dyeing or bleaching hair, which includes the following steps (a) to (d) or steps (a) to (e): (a) charging a first part containing an alkali agent and a second part containing an oxidizing agent into the main body of an openable and closable airtight container, (b) hermetically sealing the airtight container, (c) forming foam by shaking the airtight container, (d) taking out the formed foam from the airtight container and applying the foam to hair, and (e) re-foaming the applied foam on the hair.