In the Bitcoin system，transactions and their collections (i.e., blocks) are distributed over a Peer-to-Peer (P2P) network (i.e., Bitcoin network) constructed by participating nodes. Each node maintains a distributed ledger (i.e., blockchain) consisting of retrieved blocks. Therefore, speedy block distribution over the Bitcoin network is essential for all nodes to reach a global consensus on the blockchain. On the other hand, Bitcoin clients are developed as open source software, and thus they can be modified by malicious users. In the existing work, it has been pointed out that an attacker can delay the block propagation between neighboring nodes by exploiting the regular timeout mechanism for unexpected slow block transfer caused by temporal network trouble. In this thesis, we focus on a risk of block diffusion delay attack where multiple attackers colluding with a specific miner, which is a special node that aims to build blocks, disturb the propagation of blocks generated by competing miners. Through simulation experiments, we first reveal that over 40% honest nodes cannot normally retrieve a block when there are only 1% adversary nodes in the system but they are located at high-degree nodes. To alleviate the block diffusion delay attacks, we first propose a speedy recovery method from the interruption by adjusting the timeout value and give a simple analysis for an appropriate timeout value for both normal and fast block retrieval. As for a countermeasure to suppress the reception of the block diffusion delay attack itself, we further propose a block retrieval node selection method based on the past download rate from each neighbor. Through simulation experiments, we show that each node can speedily select appropriate block retrieval node in the continuous block generation and diffusion events of the Bitcoin system.