To provide a method of sealing openings, by which the deformation/ dissolution of a dipped part of a ceramic honeycomb formed body can be suppressed when it is dipped in a sealing agent, and to provide the sealing agent.

In the method of sealing the openings at the prescribed end face not to be sealed in the whole end face, which comprises coating the end face not to be sealed in the whole end face 50 of the ceramic honeycomb formed body 5 with a masking material, and then dipping the end face 50 into the sealing agent 1 while keeping the coated state, the sealing agent 1 is composed of ceramic particles and an auxiliary agent for fluidizing the ceramic particles, and the auxiliary agent is unable to redissolve a binder contained in the ceramic honeycomb formed body.